Just say 'Why? Have you finished your homework already?'
Or agree to meet him, don't turn up, then when he texts to apologise say 'ah yeah, i actually forgot to turn up....so sorry......'
or.....text back saying 'Who's that....?'
Or.....(god, i'm full of 'em......) text him saying 'oh it was fantastic on XXday night, i can't believe what time we got in, what the hell were we drinkin etc etc' then text him again str8 away saying 'oops, i think i just sent you a text by mistake'!!!