A) He died and was turned when he was 17, so in order to keep up the facade and live in polite society, he has to pretend to be a teenager and go in high school.
I've been to high school. He must be in hell. I know I was, but only for four years. Maybe I should have pity on him. He probably cracked after all this time.
Also, I was 17 before. I could pass for older if I wanted. Why doesn't he do something that isn't full of meaningless child rules? Like, get a job at the night shift at some convenient store. If one is going to spend their "life" as a 17 year old (his documents would still age with him though), why do it in the least freedom enhancing and most privacy intruding place?
B) She is 17, almost 18 (turns 18 in the second book.)...legal! And love can happen regardless of age (within reason, of course)
In that case, she's a necrophiliac.