O termo aulas livres é mais amplamente utilizado com o sentido de que você pode assistir a apenas uma aula.
Aula aberta pode remeter ao sentido de aula ao ar livre.

Sug.

I thought that 'walk-in classes' would be a more adequate translation of the intended meaning, like the original one, since I have only seen 'drop-in tutorials' so far, but I then put more thinking into it and realized that anyone can organize tutorials in a way to teach, so that it can equate in a few senses. Oh, well, drop-in tuts means that you arrive and get in, like nobody cares about who you are or what you are really doing there as long as you mingle well, like you at least pretend well that you are interested or something. I then started thinking about payments and concluded that they could demand payment if they wanted and we would still call it drop in, like we could pay for four drop-ins in six months or something, I am sure that it can be done. In this case, aulas gratis does not make sense. Aulas abertas does not make sense because the opposite to that would be aulas fechadas and we don't have such a thing. Besides, it is not really open, since, as I said, you would have to at least pretend to be interested not to be kicked out. They do have lose attendance, like nobody will mark your presence or absence in a formal way. Frequencia livre is then one of the characteristics of the drop-ins, but I think that is just a characteristic, not the main thing. Aulas livres is possible, but I think that avulsa gives more the sense of isolated whilst livres gives more the sense of no commitment. I think we mean isolated rather than with no commitment. It is like a package on its own.

uma alternativa, que me pareece mais usual em português br. E não quer dizer que é de graça, mas que a frequência é livre.

--------------------------------------------------
Note added at 1 dia4 horas (2016-07-08 01:45:46 GMT)
--------------------------------------------------

* não quer dizer necessariamente que é de graça, mas sim que a frequência é livre e não obrigatória.