First of all, if your friend's started transitioning, then it may be appropriate to start referring to them as "she".
Ask what your friend would prefer, of course, but if your friend identifies as a girl, then "she" is usually the pronoun to use.
Secondly, it's likely that this is a very big deal for her right now, so it's not really surprising that she wants to talk about it a lot, especially since she's only told you recently (and it may be that there aren't many other people around who she can talk to about it).
It's like any big life event - whether it's getting married, coming out, having a baby, converting to a religion, whatever - people often go through a phase when it's all they can talk about, but it usually wears off over time.
But you get to have your own needs and preferences as well. You've got every right to start conversations about other topics, or - if she won't switch subjects - say exactly what you're saying here: that you want to talk about other things like school and working as well.
If you make it clear that you're not saying that you don't ever want to talk about her transition, you just want to keep the other ingredients of your friendship going as well, that seems perfectly reasonable and okay to me.