There is a girl in our class (university). She knows I like her. Every time I send her a message, she doesn't answer for about 30 minutes. Sometimes I just send another one.

Explain that she doesn't want to get too close to you, and she doesn't want to reject you too directly because of classmates or other relationship problems. Girls hate pestering boys. You sent a message. If she doesn't reply to you, don't send a second message. You can try to start with some details of life. For example, on the day of her menstruation, she needed a hot water bottle to turn on the water, and she was in a bad mood before eating. It takes heart to chase girls. It's no use texting. It is always annoying to return it.