On Sunday Afternoon I just get the homework scene of Beth sucking the MC off.
i was also stuck where you are right now, mine beach event triggered after this (girl's room event).
try couple of more sundays....
before going girls room on sunday afternoon, try going to kitchen on the same sunday morning where beth ask to invite lilly.
make sure you completed the event where beth comes to your room at night.
still didn't triggered the event then go through the walkthrough and make sure you have completed all the event before this.