Unfortunately ubbu is not yet set up to handle mixed classes with different school years.
Given the way it is designed, the platform can only run one lesson at a time in each teacher account. Therefore we advise you to create two forms each with the corresponding school year, and with an associated teacher (it cannot be the same one).
To run the two lessons simultaneously, it is necessary for the second teacher to run the second lesson with his account in a different browser (e.g. one lesson running in Chrome and another in Firefox), or in an anonymous window, or even on a different computer.
Alternatively, the teacher can try to find a middle ground and select a school year for the class that is appropriate for the average age of the students. Our 3rd grade curriculum, for example, might be a good option.