Hi Bunnyhugger, toddlers are emotional little beings, with big feelings and small bodies. Sometimes it gets too much for them and they lash out. It seems as if she is trying to get your attention.
I am a big fan of dr. Aletha Solter, from the Aware Parenting Institute.
It's based on the fact that children - just like us - has emotions they must process. Be this anger, hurt, resentment, pain from ears or whatever. This is not to say that you did something that hurt or angered her but just like you have an off day or get irritated, children to too.
It's important for the children to get these emotions processed and get it out.
If it was me, I'd get to school, make a big hoo-ha of coming to fetch her. How much you missed her and love her. Hug her, pick her up. If she hits someone, completely ignore it. You love her regardless if she test you. She needs to know this. No, it is not right for her to hit but wait till you get home, let her FIRST feel completely loved. Then talk to her at home, kindly, about how it hurts someone if you hit. It makes them sad and you love your friends and don't want to hurt them. Ask her if she gets angry, can she rather tell you and she can hit a pillow etc?
Here's the
link
to Aware Parenting.
(Just know that you are not alone. Toddlers are very very hard work with their very very big egos and feelings.

)