My girlfriend sent a message in class saying, I am so sleepy. How should I reply?

You should reply to your friend's behavior very well and patiently. You must have a good class. Don't think too much in class. Although you are tired and sleepy, you shouldn't show it in class. Because there is a teacher in class, you should respect the teacher, respect the teacher's concern for you, and respect the teacher's contribution to you. So respecting others means respecting yourself, so you should do well.