He teaches ESL in our school. / He is teaching ESL in our school.
The first sentence uses the simple present tense while the second sentence is in the present progressive tense. The first sentence describes a permanent situation while the second sentence describes a temporary situation. I would help students understand the difference by describing two scenarios. The first is about Mr. Lin. He is a full-time employee at our school. He has been working at our school for five years teaching ESL. He teaches ESL in our school. He loves his job so much that he will probably teach ESL in our school until retirement. The second sentence describes Mr. Lopez. He also loves his job but he is a long-term sub at our school. He started in November. He is teaching ESL in our school. He will continue teaching ESL in our school until Mrs. Dinh returns from maternity leave. Mr. Lin’s situation is permanent so we use the simple present tense. Mr. Lopez’s situation is temporary so we use the present progressive tense.