Miss Milkie was a teacher who was mentioned (but not seen) in Recess: Taking the Fifth Grade. Not much is known about her, other than the fact she loves kids.
In Recess: Taking the Fifth Grade, just when the recess gang was hoping to have Miss Milkie as their teacher for 5th-grade, they end up getting Miss Finster. Speaking for himself and the other students, TJ (as politely as possible) questions Miss Finster on what's going on -- she explains that Miss Milkie took a job at a different school, which left her position at Third Street open, which ultimately led to TJ and the rest of the main six getting Miss Finster as their teacher for 5th-grade.