The two teams have already played each other once this season, with Whitchurch High School coming out on top with a 9-7 win.
Whitchurch High School won this cup competition in 2007, when Welsh captain Sam Warburton was in the victorious side against Neath College.
The same school-year produced cricketer Tom Maynard, Spurs and Wales footballer Gareth Bale, and rugby league international Elliott Kear. The school also boasts Olympic cyclist Geraint Thomas as one of its sporting alumni, and its football team has also qualified for the equivalent schools final this year.
Warburton attended one of the final training sessions at his old school earlier in the week, to wish good luck to a side that has won eight out of nine games this season against 1st Division league opponents:
Speaking at the Blues training centre, Wales captain Warburton said: "I played in the under 14 final and under 18 final and scored in the under 14 final, we won both, so both matches were very memorable to me.
"I loved my schools rugby because you get to play with guys you have known for about seven years and formed a really good bond with them. Playing schoolboy rugby for Whitchurch are some of my fondest memories in rugby so I'm sure the boys will enjoy it on Tuesday.
"Whitchurch were, and still are, very proactive in organising good fixtures. It does help when you play for the Blues under 16, 18 and 20 that you are familiar with a lot of the players you are playing against, who play for the top schools and colleges. It's good to get that exposure playing against teams from west Wales, the Gwent region and further north.
"I know the Whitchurch boys have been training hard in the mornings and after school. When you put the hours in and get the rewards it's very satisfying. To win with really close mates is rare as well. You don't really get it at senior level as players tend to move and not many players stay with one club for such a long time.
"I want bragging rights over Jamie so we are quietly confident of getting the win. The bond between the Blues and schools is fantastic. The region sends out development officers to help with the rugby coaching and there are ticket initiatives that benefit the schools and the Blues which helps younger kids develop rugby skills."
Speaking ahead of the game, the Team Manager for the Whitchurch side, Colin Workman said: "Tuesday will be a proud moment for Whitchurch High School, and particularly for this squad which has grown in stature and confidence throughout the season. They've benefitted from their involvement in challenging and high profile fixtures such as the St Joseph's national rugby tournament, and a hard fought 13-13 draw last month against a highly rated Millfield team."
"It's very much the team spirit and togetherness of this side that have got us this far, and all involved relish the prospect of a local derby being played at the home of Welsh rugby."
Glantaf's Rugby Manager Dai Williams - who also coached Jamie Roberts as a schoolboy - added: "We're all looking forward tremendously to this game - it's a really historic day for the school. We've had a great season, but it's remarkable that this cup competition has come down to two schools that are located within a mile of each other."
"All the games in recent seasons between these two sides have been closely fought. This is my last season before retirement, and I feel honoured to be taking Glantaf to the final once again."
The Glantaf team is coached by Dyfrig Rees (forwards) and Haydn Morgans (backs). The Whitchurch High School coaches are Steve Williams (backs) and Ian McIntyre (forwards)
Glantaf's 1st team have on the Welsh Schools Division 1 League this season - for the third season in a row. To date, they've played 31 games in 2011/12- including an unbeaten tour in Canada. They've won 22 of these games, scoring 642 points and conceding just 315. They made previous appearances in the final of 1991, before becoming the first school (as opposed to a 6th Form or Tertiary College) to ever lift the cup in 1995/96.
Jamie Roberts - Glantaf's only British Lion and Barbarian - is expected to hand out specially commissioned shirts to the players ahead of the final.
Roberts said: "I think schools rugby is massively important. It's the first step on the rugby ladder for many players who have aspirations to play professional rugby. It's important these players are exposed to basic skills and play week in week out with their mates, in a high standard of competition, which schoolboy rugby does provide.
"My advice to the player is just to enjoy the occasion as maybe you won't get the chance to play in the Millennium Stadium ever again. They need to make sure they enjoy every minute and put in their best performance because that's all you can do as an individual. Hopefully many of them will go on to play for club side in the future or representative rugby and get international honours.
"You only have to look at the likes of Leigh Halfpenny and George North, they made their international debuts at 18 or 19 and these guys aren't far off that age. I'm going down on Tuesday to present the shirts and really looking forward to do it. I always look back on my school days fondly and if I'll be a very happy man if Glantaf can get the win."
Both schools within the Blues region have a strong track record of producing first class athletes.
As well as Roberts, Glantaf has also produced international rugby players Nicky and Jamie Robinson, with Huw Harries reaching Wales "A" level, and players such as Tom Isaacs, Rhys Patchell and Lee Thomas also coming through the ranks of Cardiff RFC in recent years.
WRU partnership manager Adrian Evans added: "Schools rugby forms a key element to our 'Minis to Millennium' Player Pathway and the two schools reaching the U18 final demonstrate excellent examples of the drive and commitment that school teachers put into the development of rugby across Wales."
"The value of their work is visible not just in the development of international players like Sam Warburton and Jamie Roberts but also in the many players who go on play at club level having learned and developed their rugby at school."
The game starts at 4pm at the Millennium Stadium on Tuesday April 24th. The Dewar Shield Final for under-15 players between Cardiff Schools and the Rhondda will also be held at the stadium at 2.30pm - making it an all Blues occasion. Entrance is free