Eat your lunch while reading a book somewhere quiet like the stairwell or the corridor of your next class. If you are artsy you could draw too. If you've got exams you can revise, anything to finish off for class you could do that. Go outside, walk around the building a few times and possibly find a nice place to sit in the summer weather.
If you could try and resolve things with your friend, ask her to meet up with you in person so you both can speak about it. Being anxious is completely understandable in social situations but solving arguments over text can leave so much room for something to be perceived wrong. I know it seems like everything is going badly, whether you started the argument or someone else did but talking to them might help. As someone who has fallen out with lots of people, losing friends you thought would be there for you and vice versa long-term tends to hurt the most. Focus on developing your own character because it's super easy to say 'they were the problem, they did x, y and z'. In reality, it's hard to admit that you can be the problem and that you can be at fault. Taking responsibility is the first step to mending any relationship but depending on what happened, you also need to accept that things won't be the same. If you don't address it and just apologise, it will come back later in an argument no matter how much you try to change it or ignore it.
Take care of yourself and I am here to message if you need a friend. Summer is tough when you don't have the best relationships with others :/