An "agency" charging almost one month's of a Thai salary for working at a school...

There is only one answer: Don't do business with them, deal with schools directly.

Maybe, this agency is real and not a scam. Then, they would charge the job applicant and not the employer. There are many agencies around doing that. But even then, if you have to pay them to get a job, FORGET THEM!

Kind of a really fishy business model, and dear "Chris" signing that strange "offer" may be an old teeth/toothless crocodile hiding in the mud somewhere...who knows?