hontou ni Stripey Insists Sisters Cry Oniichan Now

from the ancient times IMO

I! My! Me! Strawberry Eggs (Reviewed 2002-03-05 with revisions 2003-11-16)

Mix of Tootsie and Great Teacher Onizuka i suppose. Guy can't get a job at a girls' school as the principal is anti-male. With the help of a rich, eccentric, techie 'grandma', he's transformed into a beautiful lady teacher for the purpose of showing up the principal. Said guy turns out to a star teacher, a hit with his/her students, ruffles the feathers of the stuffy school administration and strives to keep his real gender a secret.