Translation:The numbers of the first semester are not pretty.
English speakers would use "pretty" but perhaps not when talking about grades (mainly because I've never heard grades referred to as numbers). I have heard "the numbers aren't pretty" quite a lot when business people are talking about profits, customer numbers and the like. I suspect it's referring to numbers of students rather than grades.
I'm a native speaker of midwest US English, and I've often heard "not pretty" or "not very pretty" used to refer to financial numbers, grades, and even medical test results. I have only heard it in the negative, though, never if the number referred to is good. So while you can say "I got my grades today, and they aren't pretty," you would not say "I got my grades today, and they are pretty."
It's colloquial, but not at all uncommon, and most native speakers would understand it immediately, at least in this part of the country.
I agree that "for" is better than "of" in translating this sentence. The word "semester" is commonly used in the US to mean half a year of college, but "term" is also used, perhaps more informally, and allowing divisions other than halves. A quarter or a trimester (three make up a school year) could be a term. In primary and secondary school, I am used to hearing "marking period," which usually specifies a quarter of a school year. This word is sometimes replaced with "term," too. I am wondering how flexible the Italian word "semestre" is in accommodating these different possibilities. Perhaps schools in Italy have less variety in the structuring of the year? I also like the idea of translating "i numeri" as "the marks." This has a distinctly British tinge, but Americans hear about getting a mark on an assignment sometimes, too, though that usage seems to be getting rarer, and it is characteristically in the singular in the US. For example, "What was your mark on the paper?" As so often happens with Duolingo exercises, the lack of context allows the sentence to have many meanings. "The numbers" might mean the attendance totals for the semester, for instance, or scores on pretests, or sales at the cafeteria, or total disciplinary actions. Schools keep track of many different kinds of numbers!