I'm uncomfortable calling it laziness...more like "criminally under-addressed insecurity." He clearly doesn't not care if he can read the books enough to converse with Willow in class. Even what he'd have accidentally absorbed being sometimes ready for school would have gotten him more than what he eventually gets on the SATs, unless there was a serious anxiety issue under there (which given later events and characterization I think we can hypothesize there is). I think this is actually a pretty interesting character thread that gets dropped in service of "lol Xander dumb lol."