What is the ruling on praying Asr combined with Dhuhr before its prescribed time, due to not having time to perform it at its designated time because of studies?

You must perform the prayer at its prescribed time. You can perform ablution at home and then pray Asr at its time, even during a school break. If that is not possible and you find no way to perform the prayer at its time, some scholars have permitted combining prayers when there is a need. Thus, there is no harm for you in availing yourself of this dispensation. However, do not resort to combining prayers unless you have exerted your utmost effort to perform the prayer at its prescribed time, even if it requires changing the study schedule, if possible.