Converting a tourist visa to student visa

Hi there - have a question about my visa status.   I have a US passport and multiple entry tourist visa for Brasil.   I have used this for many years with no issues entering the country.   I will be in and out of brazil multiple times this year (right now on my second entry for the calendar year and third for the rolling 12-month period), and planning to go to portugal for a week next month.   Right now, I'm here studying Portuguese in a language school, and then I'll return to go back to school.    I'll likely do this for four weeks, then spend a week in another country, back to school for four weeks, etc.

This time, when I entered, they wrote in 90 for the number of days validity, and they had never done that before.   I think this is normal, but just wanted to make sure.   I'm also concerned that I'll be denied re-entry into Brasil in June if i'm traveling solely on the tourist visa.   

My question is whether it's advisable or necessary to get a student visa in my case, or if the tourist visa will be ok for what could be 5-6 entries in a single year?  Each of those trips into brazil will be for 30 days or less.   Also, if I do the student visa, how long does that take, and is it possible to apply while remaining in Brazil?

Thanks