I completely understand why you'd be upset but this can be a great teachable moment. Focus on that instead of getting upset with your son.
Definitely calm down before you see him, and sit down with him at home during a quiet time (not when you're busy running around the house, cooking and cleaning, etc.). Ask him exactly what happened and why he did what he did, and actually listen to him. Teachers aren't always right and children can be manipulative, and your son may have gotten in the crossfire and needs your support more than ever.
Be kind but firm about why it's wrong to resort to physical altercations, and tell him why he shouldn't do it again. Focus on the problem and the impact on him (i.e. why fighting doesn't solve anything, how it can negatively impact his future, provide alternative solutions), instead of guilt tripping him (not saying you will but a lot of parents do this and make the problem about them instead of their children). If your son was unfairly punished, this is a great time to teach him the "life isn't always fair" lesson but how you should still try to do the right thing and be the bigger person. Teach him how to navigate these sorts of situations if they happen again.