Hello Karla! I call one school in Rio de Janeiro and asked about your question. They told me that no needs a specific document to take portuguese class in Brazil for one week. It's enough your tourist's visa.

Have nice days in Rio de Janeiro ;-) 

Hi Karla.

I believe that this website will help you because it talks about Visa in Brasil.

http://www.portalconsular.itamaraty.gov.br/vistos-para-viajar-ao-brasil#precisa