The Language Institute (LI) is part of the Chiang Mai University (CMU) which is the biggest Institute for higher education in Northern Thailand with more than 20 faculties.
The LI is right in the middle of the campus next to the Doi Suthep mountain. Please click here for the location on google maps.
They offer a One Year Program called ” Thai Language and Culture” which includes a total of 216 hours. Classes are held twice a week, 3 hours each. During the year there will be two large term breaks. Those breaks last at least four weeks and are most likely during the month of April and August. Plenty of time to explore the country.The fee would be ฿30.000, to be paid upfront and is non-refundable once the course has started. The LI is entitled to support your Non Immigrant ED Visa application which you can do at your local embassy before travel to Thailand or within the Kingdom.
There are much more Thai language schools throughout town with a similar programs and you might like to surf around a bit which ones are suitable for you. Not all of them are entiled to support One Year ED Visa Application though.
The easiest way to find the Language Institute on this vast compound is to enter via Canal Road. It would be the first building to the left.