If an online language test says you've placed in B1, but advanced B1, is it better to take a B1 level course or go ahead with B2?

It depends. If you know that you can invest time and can concentrate on language with reinforcement outside of the course, go for the B2. But if you know that you will be quite busy and will need to take time to progress step by step with little or no reinforcement outside of the course, take the B1.

To take the B2, you need to be very comfortable with the subjunctive. You need to be able to write 250 words fairly correctly and quickly. You need to be able to argue a point orally succinctly which involves a good understanding of the conditional.

The difference between B1 and B2 is huge, in my opinion.

Do you have the opportunity to discuss with the school/teacher? I was in a similar situation to you and thought it would be better to go to B2, just to ask the return to B1 after a semester as it was too hard for me to follow, specially as I do not get much opportunity to practice outside of class.

sure B2, not to waste your time revising what you know

I am so with lolly on this one.

Go ahead to B2, your brain will catch up if it has to.

I took C1 courses at uni that were organized for B2 exams at the end. The profs who organized it this way knew what they were doing.