I put "races" for "corsi". Sure we've had that as a translation before. Collins dictionary agrees with me. I will report.
More natural in English: the classes will be held etc.
Bu a "course" is a series of classes. That only works if two or more "courses" can be taught in a single afternoon, which is not the usual understanding of "course" in American English.
why are they using tenersi here? I looked this up and I still cannot find the reason.
tenersi = to be held, to take place
the hints give the translation of "corsi" as a past of the verb for run rather than the noun for classes.
Trust the hints at your own peril. Even when relevant, they are usually wrong, misleading, or incomplete.
I reported the "correct" answer as unnatural or incorrect English - which it is. Nobody in the US would understand what you meant by this sentence, without some greater amount of information.
I understand it and don't find it awkward syntax. "The courses will be held in the afternoon" is equivalent to "the courses will take place in the afternoon" or "the courses will be held in the afternoons."
I would love to see more of the passive voice.