whats going to be the fastest way to level up to 80 with 10 characters

atm all are standing 61-64

options are
level death knight or paladin to 80 and then group boost each group accordingingly to 80
make 2 teams of 5 and quest to 80
make 2 teams of 5 and instance grind to 80
level them 1 by 1 (err not going to happen)

please any input would be great