"This student is studying."
Translation:Dieser Student lernt.
I wondered why "dieser" but then I found this great explanation: https://www.duolingo.com/comment/1252496$from_email=comment&
I don't know about in other English speaking countries, but in the U.S., the word "student" doesn't specifically refer to any age level. "Elementary school students" is correct. "High school students" is correct. "College students" is correct. And "Graduate school students" is correct. And any one of those could simply be referred to as a "student" without any further descriptor. "Schüler" is a perfectly good translation of "student" in the original sentence, but it's not currently accepted on 10/27/18. This confusion from the broader American English usage of "student" to the more particular words used in German comes up a lot.
So the following sentences would be right:
Dieser Student lernt. Dieser Student studiert. Dieser Schüler lernt.
„Dieser Schüler studiert.“ is not wrong, but we don‘t say that in Germany. So I would suggest the three translations above.