Young-adult science fiction novel series. Kid trained as warrior, all other kid warriors wiped out

Question

I read these books when I was kid (early 1980's) and think they were published in late 60's, early 70's or possibly later.

The premise was of a training group for teens to train them to be some kind of space warriors or peacekeepers. There were cool weapons, tactics, training, tech, etc.

There's some kind of treachery and the whole school/academy is wiped out, except one pupil (possibly one or two others?) who escapes in a ship and spends the remainder of the book series fighting the forces that killed all the other students.

Answer 1

This is almost certainly Douglas Hill's Last Legionary series. From Wikipedia's summary:

The books tell of the adventures of Keill Randor, the last survivor of his planet's population, who are annihilated at the beginning of the book Galactic Warlord. Randor's people were hardened over generations by their inhospitable planet, which (combined with rigorous combat and martial arts training) resulted in them exhibiting speed, reflexes, strength, and fighting abilities all at the very upper limit of human ability and Randor himself is one of the most skilled amongst them, twice winner of the planetwide martial arts contest, a feat bettered by only one other legionary in the history of the contest. The people of Moros acted effectively as mercenaries; however they were careful to only offer their services to clients fighting in self-defense. After they are all killed by a mysterious villain known only as the Warlord. Randor vows to avenge them.