I can tell you that unless you have been cheated on, you can't possibly understand why any woman would be wary of the situation you describe. There are a few things you need to keep in mind here. First, if you have a problem with him seeing or texting this woman outside of class or a study group, tell him that it makes you uncomfortable and you want him to keep the relationship within those boundaries. It doesn't matter why you feel this way, but it should matter to him that you DO feel this way. And he should immediately agree to it because you are his wife and YOU are the one he needs to work at keeping happy.
Second, wives get to be jealous. In a marriage, you give up a lot of privacy. Grab his phone and check it out. Do some snooping without him knowing. If you find something suspicious, sit him down when you are calm and talk about it. Give him the benefit of the doubt and let him explain. If his explanation isn't good enough for you, then maybe you need a neutral party (a counselor or your minister/priest) to help you work through it together.
Third, don't panic :) The key is to present it to him as if you feel like you aren't "part of the team" anymore, that you feel like he's hiding something or she's filling up a place in his life that you are suppose to be filling. And as his wife, YOU get to set the boundaries for his relationship with women you are suspicious of.
My husband has a saying, "Always date your wife." If he's not still courting you, he should be made aware of that and the two of you need to make more of an effort on each other.
And yes, 80% of people admit to cheating on a partner as one point or another. No, not all men cheat, but the majority of them at least think about it. Any woman who tells you otherwise is fooling herself. Monogamy is HARD, it's not easy to stay married for 50 years, it takes a lot of work and dedication. But most of all, it takes communication. You won't know anything until you talk to him. Start with what you said in the last line of your post. Remember, you two are on the same side :)