In my experience the most common idiom is a scheduling conflict This can apply anywhere, not just to academics.
For example, in a work email: Hi Jim, can we move our meeting to 3pm?
I have a scheduling conflict with another meeting at our original time.
Thanks.
Note this assumes you want to participate in both events.
If these classes just happen to be at the same time, then Laurel's answer of "simultaneous" is more accurate.
ell.stackexchange.com/users/42184/Andrew
I had this happen to me a lot at University, and the word in use then was 'clash'.
Oh no, I have another timetable clash this semester.
or I have to see my tutor;
plant biology clashes with statistics on Thursdays
ell.stackexchange.com/users/103741/flashliquid
The classes are simultaneous : occurring, operating, or done at the same time.
— Lexico/Oxford Dictionaries
Here's an example in use (from a tango site ): Saturday July 25th 1-2:15pm w/Anais - Beginner Level - Embellishments for the leader and follower 1-2:15pm
w/Carlos - Intermediate Level - Paradas/Barridas (above classes are simultaneous)
ell.stackexchange.com/users/41273/Laurel
From what I understand, you are saying The "Microeconomics - ECON101" class is scheduled at 11 am, and so is "Introduction to Psychology - PSYC101".
Is that correct?
Well, may be the easiest way to say this would be Both Econ101 and Psyc101 classes are scheduled at the same time.
You could also say ...
my Econ101 class coincides with my Psyc101 class ... From Cambridge (1) and Collins (2) "coincide" means (1) to come together in position or happen at or near the same time (2) If one event coincides with another, they happen at the same time.
There are many ways to say this
(you have not provided an example sentence)
You could use the word "concurrent" meaning "happening at the same time"
(Cambridge) ...
Econ101 and Psyc101 classes run concurrently ...
As Andrew mentioned, you can also say There is a scheduling conflict [or schedule conflict] ...
A scheduling conflict occurs when two (or more) subjects are offered at the same time, and the student must make a choice between the two. - How to Build the Master Schedule in 10 Easy Steps (2008).
ell.stackexchange.com/users/77766/AIQ
I would express it by saying that the classes are "in conflict."
Oh no!
My math class is in conflict with the English class I wanted to take.
Or... That job would be a conflict with another commitment I have.
ell.stackexchange.com/users/103720/General Nuisance
Overlapping schedule is not a bad phrase.
(Furthermore, neither are some of the other good answers.)
That is certainly a phrase that is used.
Although, if you know exactly how many items are having overlapping schedules, it may be a bit more common to say a more specific term, "double-booked " or " triple-booked ".
ell.stackexchange.com/users/17653/TOOGAM
If you are scheduled to be in two classes (or appointments in general) at the same time, you'd describe yourself as double-booked .
ell.stackexchange.com/users/4675/Brian Minton
For example, I need to enroll in two different courses at university, but there is a schedule in which both are held. Is it an 'overlapped schedule' or something similar?Is there a proper expression for this purpose?
What is it called when at university there are two subjects being held at the same time?
ell.stackexchange.com
<phrase-usage><phrase-request>
ell.stackexchange.com/questions/228384
ell.stackexchange.com/users/103718/Felipe Varas
The most common idiom is a  scheduling conflict   This can apply anywhere, not just to academics.   You could use the word  "concurrent"  meaning "happening at the same time" The following possibilities are an option: "a scheduling conflict"; "classes are simultaneous"; "classes are clashing"; "classes are overlapping"; or "classes are double-booked".
The following possibilities are an option: "a scheduling conflict"; "classes are simultaneous"; "classes are clashing"; "classes are overlapping"; or "classes are double-booked".