You're being praised. The proper response is Thank You.

You may say: I will work more hard.

You may not need to reply at all. Your response depends on the context and who spoke first, you or the teacher. Example 1: (class activity has just completed; teacher speaks last) You: That was fun! T: I'm glad you enjoyed it. (no reply is needed or appropriate) Example 2: (you arrive late to class due to very bad weather; teacher speaks first) T: I'm glad you arrived safely. You: Me too. (or thanks)

A little more context would help, but I'll give it a try. I'm glad you are enjoying learning English. Yes, I am enjoying it, thank you. (You could leave the "thank you" off but natives wouldn't) I'm glad your house didn't get swept away in the flood. Thank you, I am too or I am too, thank you. I'm glad you managed to show up for class on time. (weak smile as a reply) Hope this helps.

you can always reply by a smile :pp