Wales will now play Fiji in the Cup quarter-final tomorrow after the south sea islanders finished second in their pool behind New Zealand.
"The squad did well, beating Tonga first up, and then showing great character to come back from 12 points down with two minutes to go and beat Scotland," said Wales head coach Paul John, before quipping, "Alex Walker kicked the conversion to win the game - all the practice with Byron [Hayward] over the last few weeks paid off!"
"That win set up a game against England to win the group. The boys played very well against England to win 21-7. The young players really took the game to the them and were very threatening with the ball - Harry Robinson scored two tries and Owen Williams (Blues) scored the other.
"The boys defended really well and worked hard for one another, every member of the squad made a valuable contribution to our performances on Day 1. We now need the same on Day 2."
Wales began the day slowly, with Tonga level pegging at halftime 7-7 with Jack Ram and Will Price trading converted tries.
But it was all one way traffic in the second spell with Price adding a second before Rhys Shellard and Owen Williams finished the job off.
Scotland gave Wales a real scare before Richie Pugh took advantage of the Scots being a man down after Andrew Turnbull was sin binned. With time rapidly running out, Pugh raced over in the dying seconds to put Wales in front for the first time of the match to scrape home a 14-12 victory.
Scott Riddell's try had given the Scots a 5-0 lead at the break and when James Fleming scored early in the second half Wales were staring down the barrel of defeat. But a try by Owen Williams got Wales back into contention before Pugh intervened to confirm the win.
The crucial clash against England was an evenly match encounter in the opening exchanges. Owen Williams raced over for Alex Walker to add the extras but the red rose brigade hit back through Mat Turner with Marcus Watson knocking over the conversion.
Cardiff teenager Harry Robinson then gave Wales the perfect lift by scoring just before the break. With Walker adding his second conversion, Wales held a 14-7 lead at the break.
Robinson put the match beyond England's reach when he scooted over in the opening minute of the second half. Walker's third successful conversion put further daylight between the two sides.
England threw everything at Wales but the Welsh defence held firm as climbed to the top of Pool D to earn a shot at Fiji in the Cup quarter-finals.