In my opinion, working for an agency is one of the worse things that can happen to a teacher in Thailand. Unfortunately, many schools in Bangkok are staffed by agencies. Here are some things you should keep in mind before you decide to work for an agency:
1. Agencies charge a minimum of 30% of your earnings. In some cases it is as much as 50% or more. If the school pays 60,000 baht per month, you may get 35,000 baht.
2. Schools pay for the whole school year, so if the agency offers a 9, 10 or 11 month contract, it actually takes even more of your money. If you don't get paid for semester breaks, it means someone took your money.
3. Probationary period can be a total sham. It takes a maximum of 2-3 weeks to organize the documents needed for a Non-B visa, not 3 months. Also, you should get a "work permit" as soon as possible, not after a few months. Working on a tourist visa or visa exemption is illegal.
4. You are just a warm body in the classroom. In case of any problems the agency will always side with the school and not you. In case of any problems with visas etc. the agency will always say that it is your fault.
5. Some agencies provide educational materials (e.g. PowerPoints), but most often they are of low quality, with many errors.
6. Many times you will be lied to by agencies. Their goal is only profit and that you quietly make hours, not your welfare. Some agencies portray themselves as "cool" and "trendy," but they too don't give a damn about you.
Do your best not to have to work for the agency. Good luck!