Call her out of course!

How embarrassing would it be to continue walking around the classroom knowing very well that your lab partner is staring at your ass the entire time! It'll probably be embarrassing for Hateya once she realizes you're onto her, but it beats the alternative...

"Hateya, I know-"

"I'm examining your glutes? I'm well aware you do. Just testing to see how long you would be willing to continue." The scientist startles you with her casual response to the subject. She writes in her notebook some more while continuing. "It's fairly difficult to see anything with your clothing impeding the view, but I already deduced that a hormonal teenage male is far too self conscious to expose his bare posterior without any sexual intent."

"W-well yeah, obviously! Why are you even...examining them?!" You yell in a hushed whisper, careful not to let your distracted classmates hear this humiliating conversation!

The sound of led scraping against paper was the only reply you got for a while. Eventually Hateya finally came to a halt and put the notebook down entirely, turning in her stool to face you.

"I am running a vital personal experiment. In all honesty, this is the first time I have been partnered with a man in the class of my dominant subject. The longer I spent in close contact with you while studying your mannerisms, the more interested I became in the difference of behaviors between our genders. The male specimen is different than the female in various ways. I'm curious to learn more about the psychological as well as the...physical differences through personal experience rather than just the internet, where all of my current knowledge resides. Hence why I was studying your derriere while also testing your limits when being so closely examined by a female; a psychological and physical test being done efficiently all at once."

"Although I do believe that my curiosity stems also from my heterosexual interests in the opposing gender. I am extremely fond of the view that is the male 'butt', mostly in regards to the muscular structure and shape that makes it so different from my own or another woman's. There's something alluring about the sight that tempts me to learn as much as I can about the looks and feel of a man's bottom. I am of course also interested in the pectorals, shoulders, genitals, and everything else involving the male anatomy. I simply have my preferences just as everyone else does."

"Wait, wait, wait. Slow down!" To speak so quickly and clearly was a gift that only Hateya could have. All that information coming down at once didn't even process fast enough for you to be embarrassed! No other girl in this school could possibly speak this bluntly and casually about this subject! "What kind of experiment would you even need to know all this for?"

The teen nods. "Ah, my apologies. The experiment I am conducting outside of our classwork is..."