Regarding this that option is not considered in the calculator since you don't be charged on the amount of users allowed. The VM instance is charged on a basis of how much resources in regards to CPU, Mem and Storage will be used with the VM instance .
In windows VM instances by default the limit is 2 unless you acquire CAL license to increase the number of concurrent connections. However if you going to use linux VM instances that limit is not set by default or to be more properly will be the limit is determined by the sshd_config file, where you can change it too. I would like to mention that the limit in linux could be affected by the resources, I mean depending on the amount of users, you have to increase the resources of the VM instance (vCPUs and Mem for example), thus this will directly affect the cost of the desired VM instance deployed in GCP.
Most likely, if there is someone else who has tried or is currently using it. In my personal, I did already try to do similar situation with 15 concurrent users and all were working fine. You can try starting looking carefully on what are going to need (the students) to decide the machine type or to test with the lowest configuration possible and scale up depending on the behavior shown.