Can a dictionary key be an integer?

[buran]

Well, openpyxl guess the type and in your case in the input file student numbers are actually numbers. That is visible from the example in post#5.
Now it is clear that in your target file student numbers are text. Note single quotes in your example. And that is something new that you didn't mention so far...
If I have to guess for some reason the cells were formatted as text.