This could be a life-changing summer as you (perhaps for the first time) clearly see your father's limitations. When he said that you're “falling below his expectations,” he's speaking from his own ego. When you perform well, he feels better about himself and, when you don't, he feels worse about himself. It's all about him. Perhaps, that's why your mother and he are no longer together.
Sadly, he's making a common critical mistake of divorced moms and dads: criticizing the other parent. That puts you in an awkward position because, of course, you want to remain loyal to your mom. After all, she's the one who's with you most of the time, putting in the time and effort. It's easy for your dad to criticize when he primarily remains on the sidelines.
The writer, Maya Angelou, said: “I've learned that people will forget what you said, people will forget what you did, but people will never forget how you made them feel.” If your dad is making you believe that there's something wrong with you, he's not acting in your best interest. The time you two have together this summer is precious, and he should be using it to get to know you better, listen to you, and create special memories.
I'd talk to him about your feelings and tell him why you're thinking of leaving. Listen to what he says, notice if he apologizes, and watch if his behavior changes. He might see the error of his ways or he might not. But, at least, you will have spoken your truth and given him an opportunity to alter course.
If things stay the same, discuss it with your mom. You don't want to stay trapped in a bad situation. You want to take control. Best to you!