Learn Italian: Il professore non legge le tesi degli studenti. - The professor does not read the students' theses.

Tesi is an invariable noun, meaning its form doesn't change between singular and plural. You can tell it's plural here because of the feminine plural article le. If it were just one thesis, the sentence would say la tesi. Many nouns of Greek origin ending in -i behave this way.

Is tesi a feminine noun? The -i ending usually makes me think of masculine plurals.

Yes, tesi is feminine! This is a common trap. While most Italian words ending in -i are indeed masculine plurals (like i ragazzi), these specific Greek-origin words ending in -i (like tesi, crisi, analisi) are almost always feminine. That's why it takes the feminine plural article le.

Why is it degli studenti instead of just dei studenti?

Because studenti starts with s + a consonant (t). For masculine plural nouns starting with this sound (as well as z, gn, ps, and vowels), Italian uses the definite plural article gli. When you combine the preposition di (of) with gli, they merge into the articulated preposition degli.

Does tesi only mean a university dissertation, or can it just mean an argument or theory?