What are the requirements for getting a 90-day extension? Or are there no requirements, other than showing up and paying? What is the maximum extension allowed on an education visa?
My understanding of the ED visa is that you can get an extension of stay for 90 days or for a year, but that you also need relevant documentation from the school that you are attending. This can be done (at a cost of 1,900 THB) from within Thailand - there should be no need to go out of the country to get a new visa.
These visas have suffered from some abuse in the past (people using them to stay in Thailand long term but not then going to the school, for example), and so there are additional requirements such as showing an income coming from outside Thailand, or sufficient funds in a bank account. This is supposed to stop people from working in Thailand but staying on an extension of stay based on education.
There have also been cases where people have stayed in Thailand for long periods of time on 'education' extensions of stay who claimed to be learning the Thai language. When applying for the next extension, these people have been asked questions in basic Thai by the immigration officers, and if they could not hold even a basic conversation they have had their extension refused.
For the latest information about visas and extensions of stay in Thailand, I recommend reading the Thai Visa forum.