Katerina, good afternoon
I would like to answer that you have already found an easy and convenient platform, just click "Install OneBox" on the top right and try to figure it out yourself, maybe with the help of the Documentation section. Up to the first 1000 entries, the system is free, without functional limitations (some applications require separate payment).
However, we know from experience that each business attaches its importance to these concepts.
From a technical point of view, OneBox is a large designer that allows you to flexibly adjust it to your business processes. It will be much easier to evaluate the same when you can describe exactly what business processes you have, how they are interconnected, what mandatory fields, appearance, rights and restrictions for employees and everything that is important to you.
If it is difficult to describe, then we can help clarify all the requirements and document them in a form understandable for integrators at the rate
https://integrator.com.ua/stvorennya-dokumentatsii-tekhnichnogo-zavdannya/, the number of hours will depend on how long it takes to collect all the requirements.
Or state your budget and we will try to estimate what can be further customized for you, including individual training and instructions.
*for example, they configured the class schedule, business processes Registration and Education, finances, the student and parents' office, viber bot for registration and payments, etc.