At the dinner table, Wally says that the Boothbys have "one children" and that it's a girl. When Wally is being picked up for their date, Evelyn introduces her sister to June and Ward. Wally is clearly familiar with the sister, so he knew at the dinner table that there were at least the two girls in the family.
Wally is now a high school junior, yet during the closing credits he is wearing his freshman letterman sweater with one sleeve stripe, not his junior sweater that would have three stripes.