Hogwarts has changed since the final battle against Voldemort. Numbers are considerably smaller and a period of rebuilding had to take place. The main differences are as follows.
Grades replace Years - Because students of all ages will have come to the re-opening it would be unfair to start the older ones ahead of the basics they don't know. To fix this Grades have been put in place and are based off ability and knowledge instead of time spent at the school. If a student passes their Grading Test they will move up a grade, whether or not it's been a full year. Grades run from First to Seventh.
Students are assigned Mentors - To help students settle in they are assigned mentors who are usually Teachers. In time Sixth and Seventh Grade students will also become mentors. This helps promote writing between Teachers and Students, and between different Grades/Houses.
Notable Teachers - Currently the Headmistress is Minerva McGonagall, and the Herbology Professor is Neville Longbottom. These are currently not written by anyone and will likely stay NPCs. Your character will have reason to have heard of them and their efforts in the war.
Notable Students - Harry/Ginny's and Hermoine/Ron's children attend Hogwarts but are currently not written by anyone. If you'd like to write as one just ask.