Is there a way to write tablature without having rhythm values?

You wrote:
My students use their ear to hear how long to hold the note...

Consider this...
Is the TAB score intended only to be played by one student (at a time), subject to how each individual "hears how long to hold the note'? Therefore, each student will play a (technically) different rhythm.
On the other hand, if the students will play the TAB together, how can you be sure that each individual student in the group will "hear how long to hold the note" and that all notes will be sounded in synchrony for each student? After all, they all do not use the same ear.

Now, to address your question:
Is there a way to write tablature without having rhythm values?

If you don't care about playback within MuseScore, you can select the entire staff and use menu item: Tools > Measures > Join selected measures. Then enter all TAB notes as quarter notes. (You must at the very least choose some duration to enter notes.)
I have attached a MuseScore file to this post. This image is from that file:

It's a phrase from the song Jingle Bells - the part that goes: "Jingle Bells, Jingle Bells, Jingle all the way."
As you can see, there is no rhythm - just a series of TAB "notes".

However...
If you still want some semblance of recognizable playback - without showing explicit rhythm values - look at this:

MuseScore provides a range of tablature types.
You will find the terms "simple", "common" and "full" in the tablature staff type names:
Simple: Displays just fretmarks.
Common: Displays fretmarks, rests, stems and beams.
Full: Displays fretmarks; rests, stems and beams, half-note stems.
See:
https://musescore.org/en/handbook/4/customizing-tablature-staff#change-…

When you open my attachment in MuseScore, if desired, you can hide the guitar standard notation by clicking the "eye" icon in the Instruments panel for that guitar.