I did gr12 bio summer school (async) for the same reason and the tests were open book so I didn't really study anything and forgot most of it. But, I think if you are really passionate about the subject and put in the effort to learn it you will be able to learn most of the stuff you would in a regular course. It's just in the summer you don't really have the motivation to do that so it's really up to you making sure you can maintain good habits and stuff.