There are a number of ways to find the names and addresses of schools.
If you read Spanish (or know somebody who can help you), you could look at the on-line telephone directory, yellow pages (paginas amarillas). If you speak Spanish, you could also call telephone enquiries.
You could also try contacting the British Council in Mexico. In some countries, the British Council holds names and addresses of language schools. If the one in Mexico doesn’t have this service, they might be able to direct you to an organisation that does.
You could also contact chains of schools; for example, International House, Berlitz to find out if they have schools in Merida.
As Merida is a fairly large town, you might be able to get in touch with a library or tourist information centre. For the library, it might be necessary to speak Spanish.
You could also try the Ministry of Education, but I think that is a long shot and probably time-consuming.
I hope this will help you.