Can I say 'co-student'?

Solution 1:

The more usual term in the UK would be fellow student. Academics who work together are colleagues.

Solution 2:

How about lab partner? That would imply the two of you are working jointly. Research partner might work, too.

Solution 3:

They are your peers not your co-students.

Solution 4:

When I was a student, we referred to students who shared our lab as lab mates, and those who shared classes as classmates.

Lab mates were specifically peers. Lab members could include anyone from techs to profs.

Solution 5:

• as to it already existing, the word 'co-student' does not show at all in google books and 'costudent' is very rare

• it is not a terrible sounding neologism and would be understood with no problem, but there are other existing terms that are sufficient. In comparison to 'fellow student' it is rare.

So it all depends on what you want to emphasize.

• If it is the education part, then classmate or fellow student

• if working, whether fulltime or not, then colleague or co-worker. In your particular instance, lab member, or group or team member, would also work.

• 'costudent' is not recommended, since it is not very common. A plain google search shows that it is used sometimes (COCA shows nothing), but it looks like telescoped 'headlinese'.