My uncle who would like to take Thai classes would like to enroll at a legitimate Thai language school and where he would not have to renew the visa at immigration every 90 days. Does anyone know such school? 

You might get a bit more of a response if you actually told people

where your uncle is???? Is he even in Thailand??

Guessing he would study in Thailand given the comment about 90 days and immigration. I don't think you can get out of doing that. 

21 hours ago, phuketjock said:

You might get a bit more of a response if you actually told people

where your uncle is???? Is he even in Thailand??

yes my uncle is in Thailand

36 minutes ago, drronnie said:

yes my uncle is in Thailand

Where in Thailand does he need that school to be?

Language schools fall under the classification of being a informal school. That is why only 90 day extensions can be granted by immigration up to a year of total stay before needing to go out for a new non-ed visa.

If he was to enroll in a university to study Thai he might be able to get one year extensions.