How can I make sure that just because I don't have time to be "go out" with my coworkers, I don't lose my job?

I have two jobs, and only work part time at a private academy teaching English and Spanish. I am a full-time content editor and usually don't have time to always go out with my coworkers at the school, but they've stopped specifically asking me to come along to lunch dates with them. There is a short break between the summer and fall session and the fall session starts tomorrow and I have called my boss to get my schedule. She refused to pick up my call and waited until I hung up only to text me back that she hasn't made the schedule yet and that she was on vacation. I get along well enough with one coworker and she told me she hasn't received her schedule either, despite work starting in a few days. They told me this was because since it was a private school, the first week, there usually are no students because they're just starting school and probably don't know if they need any supplemental classes at the academy, yet.

I just noticed that all of my coworkers were tagged in a vacation picture in San Francisco. I wasn't invited and my boss was in the picture. A few weeks before, I was forced to defend myself against rumors that I was talking badly about the receptionist behind her back to my students. I asked which students had said this and she told me it was the students from my class during the second period. I don't have a class during that period.

I haven't done anything wrong. I'm the only one on the staff who has an English degree and speaks 2 other languages fluent enough to teach it, but would not being social enough cost me my job? When I had first started everyone was nice enough, but it was only this summer when they constantly went out together and I had to work my other job that they formed this tight knit group I wasn't a part of.

How can I confront this without seemingly causing trouble, and how can I get through this and get along with my coworkers?