in order to apply for the "change of visa type" to a 90-days Non-Imm-ED visa, you will need all the relevant paperwork from the school ready to show Immigration. There is no guarantee that Immigration will accept it . . Check the requirements here, Your situation is in number 3, just click on it
5000.- THB is the standard price, and it goes without any tie-in deals like life-insurances or accident insurances, some banks want you to take up when you need a bank account with them. BEWARE!
of course there are agents in Bangkok, for the bank account and drivers license service. However I do not know any addresses, you have to google for them
this is immigration specific. Some immigrations allow you to skip the new registration when you return from inland travel in Thailand, others don't. In most immigrations however they insist that you register anew after returning from an abroad trip. Visit your immigration and ask how they treat it
your problem is, in the first year of the Non-Imm-O/A visa, you cannot cancel it by exiting. It is a "multi entry" visa type. you have to sit out the whole year until the visa validity has expired, Exit shortly before and re-enter after this date.
oops, sorry, I forgot the bilateral agreement with Brazil was extended from diplomatic passports to ordinary passports. YES, you will get 90 days, and you can do at least one border run that will get you another 90 days admitted stay