MMA is a day school, so nobody will be there if you attack on a weekend. I suggest you attack when the students are there and the professors are there, during 8 a.m to 4 p.m, Monday through Friday. But do not worry about the battle itself—we are Purebloods, whereas the "professors" at MMA are all mudbloods (except for one blood-traitor who teaches Arithmancy)—your Pureblood followers cannot fail to defeat those lesser magicals.