One thing I learned during my graduate program/postgraduate programme

Although it is important for teachers to prepare their lessons thoroughly, they should be flexible in their teaching as well. Likewise, while students need to be conscientious about their research, they shouldn't be obstinate about their reseach topic and should be willing to change it if necessary.

Additional Notes:

-I changed classes to lessons, because typically it is lessons that are prepared and classes are taught. Depending on your meaning you can say "prepare their lessons" or "teach their classes"

Hope that helps!

One thing I learned during my graduate program/postgraduate programme