Goofs

Marcy places her valentine in the box, then in the very next shot, she is standing across the room in front of Sally.

Towards the end, when the herd of kids exits the school, Peppermint Patty and Lucy can both be seen multiple times in the group.

Sally is the younger sister of Charlie Brown, and Lucy is the older sister of Linus, so they shouldn't be in the same class (or grade), as they are shown here.

Peppermint Patty, Marcie, and Franklin attend a different school, so they should not be at the Valentine's Day party.

Miss Othmar leaving her students unattended while going off with her boyfriend would not be acceptable in a school. She would have another teacher take over the class before leaving.