ok in my class the highest was 21/25. there were a couple of 19s and (hehe) i got 16 (but i get what i did wrong so its ok). not sure what the average was... i think sir said a lot of them were from 15-11.
but hist ext 1/2 yearlies dont really mean anything, they're just a practice to see what you need to improve.
(and 16 aint that great so knowing it ends up as a mark out of 2.5 is very reassuring.)