Yes, Sevilla is too hot in summer for language classes and teenagers (!!), unless you can find an apartment with swimming pools and gardens for your stay in Sevilla. But even, on Sundays at that time, the city is deserted as everyone is out at beaches or mountains.
In my opinion you ‘d consider some Spanish courses 1 hour from Sevilla, by the sea, in Sanlúcar de Barrameda (Cádiz), el Puerto or Cádiz city.. Check this site
http://www.valcia.es/ If your option is northern Spain, I’d bet for Santander: animated, touristy and cooler in summer.