In British English, your mates are your
friends.
You can also refer to someone's schoolmates or schoolfriends, but
in this case the two words don't mean quite the same as each other.
Schoolfriends are actually friendly with other and like each
other, as the Bank of English shows:
His school friends all live in Rye and he has to stay overnight
with them if he wants to see them out of school.
Patrick has become even more mischievous since he started school,
especially when his schoolfriends come to play.
She was going to see a movie with a schoolfriend.
But the same is not necessarily true of schoolmates, as you can
see in the following examples:
I hated my schoolmates.
She found lessons boring and her schoolmates cruel.
He had been forced by his school-mates to eat grass.
I've picked the most extreme examples here, just to make my point
clear; schoolmates are simply children who are in the same school or class, whether or not
they are friends. Your schoolfriends are also schoolmates, but not necessarily vice versa.
However, both words are commonly used to talk about people who
were at school together in the past, even though they have now grown up and left school:
He is very loyal to old school friends.
...Dr Maurice Morton, a former schoolfriend of Mr Jameson.
He left home at 22, bought a ramshackle maisonette in West
Norwood, south London, with John Irwin, a school friend with whom he still writes comedy
material.
...Doris Parry, another old schoolmate, though not as close as
Mary and Susan had been.
He started the magazine with an unemployed former schoolmate.
Some of his schoolmates were businessmen.
In some of these examples, you find the words old and former, but
not in every case.
