English Grammar
I always feel confused about these two phrase: think of and think about.
so I really have no idea which one I should use in this context.

Here is the context: my professor had not shown up even though 15 minutes had passed and we were all feeling a bit worried because we had a mid-term test that day and hoped we could end it as soon as possible but my professor had not entered the class even though it was time to show up so I made a joke with my friend and said: "maybe she only just started preparing for the test and just thought about what she wanted to test us", but I don't know if I have used it right or not, so I asked you guys here to clear up my doubt.

A.)Maybe She just thought of what she wanted to test us.
B.)Maybe She just thought about what she wanted to test us.

Question

English Grammar
I always feel confused about these two phrase: think of and think about. 
so I really have no idea which one I should use in this context.

Here is the context: my professor had not shown up even though 15 minutes had passed and we were all feeling a bit worried because we had a mid-term test that day and hoped we could end it as soon as possible but my professor had not entered the class even though it was time to show up so I made a joke with my friend and said: "maybe she only just started preparing for the test and just thought about what she wanted to test us", but I don't know if I have used it right or not, so I asked you guys here to clear up my doubt.

A.)Maybe She just thought of what she wanted to test us.
B.)Maybe She just thought about what she wanted to test us.

Answer

Thank you guys

Answer

If you are conveying someone's thoughts that originally sounded 
I want to test them
He is a good guy
My friends study English
you'll say
She thought that she wanted to test us
She thought that he was a good guy
She thought that her friends studied English
You can change the sentence so that it has the prepositions
She thought about/of testing us
It won't be as simple with the other two sentences.
As for the difference between the prepositions, you can read this material
https://english.stackexchange.com/questions/71003/difference-between-think-of-and-think-about

Answer

@Adam
I guess the asker means She just thought THAT she wanted to test us))

Answer

Kai, I believe we need more information to answer this question.  In different contexts, each sentence could be used, especially if it is part of a conversation.
The sentences seem to be missing something.
"Maybe she just thought about what she wanted rather than what we all need to be thinking about right now, just to test us".  or "Maybe she just thought of what she wanted as the goal for the group, rather than making a group decision to suit us all, just to test our patience with her".
I have tried to give a more authentic context, however it may be different to what you have in mind.