I actually talked about this series pretty recently (and, looking at the post I talked about it in, I get the feeling this post might end up exactly the same way), but I'm still going to put it out there. I really love this series. And there are SO MANY ALIENS. She dates an alien in the first book. She gets adopted by an alien race. She heals aliens, she lives with them, she fights with them. You almost forget that she is human, there are so many aliens in this series.
And they're all described so well. Viehl's strength is probably in creating fascinating alien races that are individual, with individual looks and strengths, and weaknesses, and languages. Part of what I love about this series is not necessarily the characters, but the world building that he does. I don't read a lot of sci-fi, so for me to recommend a sci-fi series twice in one year probably means you should go read it.
I loved the My Teacher Is an Alien books when I was in middle school. And it wasn't just because they are fun, silly, books... I thought it was just the coolest thing in the world that my 6th grade math teacher's math teacher wrote them. (I also thought my 6th grade math teacher was really cool. Shout out to Mr. Tyminski!)
I don't remember a lot of the details of the books, but I remember loving them. Another teacher, or maybe the principal or something, she's an alien too. I think the teacher's house was the spaceship? And there was a little fuzzy squishy creature that I always pictured as something like a Tribble. I seem to remember a scene similar to that one in Lord of the Rings where Merry and Pippin are separated (except with middle school students, and space, instead of hobbits and Middle Earth).