I am a Mac user, and I may not be able to help you.

1) Please specify the steps you’ve already taken (to solve the problem), so that we can rule out things.

I suppose that you have already tried a reboot (?)

2) Your are not talking “Finale-speak” - right?

There are several expression categories in Finale.

One of the expression categories is called Dynamics. The category Dynamics includes expressions for the “loudness”, such as ff, f, mf, mp, p, pp.

Another expression category is called Tempo Alterations. It includes expressions such as accelerando, rallentando, &c.

The About Finale option in the Help menu on Windows will tell you what version of Finale you are using. Since your version does not appear to be behaving the way your teacher's version does, knowing what version you are using might be helpful.

Ok so to answer your questions, I have tried a reboot and I reset my preferences to default. 

Finale is ignoring my tempo alterations. 

I'm using version 26.3.1.520

… If I send the file to my teacher, it works fine on his MacBook, but there is zero difference in speed on my pc …

Since the Tempo Alterations are performed on your teacher’s Mac, but not on your own PC, I suspect that

- the Tempo Alterations are not defined for Playback,

- Human Playback is toggled on on your teacher’s Mac, but toggled off on your own pc.

(just guessing)