Hola Shirley

I understand why you want the relative in the singular form as it could be referring to "la autoescuela", but in this case, it's referring to the "sesiones", so, if we see the sentence prior to this, it says:

Me toca reservar otras tres o cuatro sesiones más en la autoescuela en las que pueda volver a repasar...

I have to book another three or four more sessions at the driving school in which/ during which I can go back to revising...

This is why the plural is used, using as a referent: sesiones (they want to revise in/during those sessions: en las que...)

I hope this clarified it.

Saludos

Inma