Is that I won't have anyone to sit with during lunch.
Gah! Why did all my friends happen to have second lunch? D:
You can sit with anyone you want. Provided they don't mind company.
Funny thing about kids, that carries on into adulthood. People can be fearful of the unknown until they learn not to let it bother them. While you can't control others, you can do something about yourself.
Are you afraid of people you don't know? If so, why?
Is there anyone that has the same lunch as you who is in any of your classes? If so, this is an opportunity to get to know them better.
Do you yearn for topics to talk about besides classes? Find a way to start a conversation with something that interests you. Maybe some video game, or hobby you're passionate about. Of course, you can always talk about boring things like .. class, the weather, or the food.