“Attendance is now compulsory for every young witch and wizard,” he replied. “That was announced yesterday. It’s a change, because it was never obligatory before. Of course, nearly every witch and wizard in Britain has been educated at Hogwarts, but their parents had the right to teach them at home or send them abroad if they preferred.”
Deathly Hallows, Chapter 11 (The Bribe)
Was it still required by law to go to school after Voldemort died?
Although it doesn't prove it, it should be noted that Harry and Ron did not return to school after Voldemort was killed. They haven't been known for their rule-following in years past, but you'd think they would want to complete their education if it were compulsory. Instead they just grabbed jobs and settled down.
Here's a quote from the wiki article on Hogwarts:
Some students, such as Hermione Granger, returned to the school in 1998 to complete their education.
It seems that it was not compulsory to attend Hogwarts under McGonagall. This is logical, as any likeness to the old Hogwarts under Voldemort she would want wiped from the new Hogwarts.
Attendance at Hogwarts stopped being mandatory after the Dark Lord’s death, since JKR says that though Hermione did, Harry and Ron both never graduated from Hogwarts and instead went straight into working for the Ministry. If they were breaking the rules, presumably they couldn’t be hired by the Ministry, as the Ministry is responsible for enforcing the laws of the British wizarding community.
JKR: Yeah, did they graduate from Hogwarts?
SU: Yes, did they?
JKR: Harry and Ron didn't go back, Hermione did. Did you bet right? You must've, I mean, come on. No one's gonna think Hermione wouldn't go back.
SU: I predicted, yeah.
JKR: Of course she'd go back. She has to get her N.E.W.T.s. Ron was really done with schooling. It would be kind of tempting to go back just to mess around for a year and have a break, but he goes into the Auror department. He's needed. Anyone. Anyone who was in that battle on the right side, Kingsley would want them to help clean up the-- I mean, anyone who's old enough to do it, who's over-age. But Kingsley would've wanted Ron, Neville, Harry and they would've all gone, and they would've all done the job.
- Interview with J.K. Rowling (17 December 2007)
As Harry and Ron both got careers in law enforcement without having graduated, attendance at Hogwarts must have stopped being required by law after the Dark Lord’s death.
