Refusal to join the reserve forces

I would like to consult with you, I am supposed to start two tough courses at the Open University in the summer semester, and I was accepted into the reserves last Friday.. If I stay in the reserves until then (we have an open order for now)
To date, I have reported for all the reserves, but this time I don't think I will be able to combine them, and postponing these courses will harm the continuation of my studies. Of course, the reserve is limited and if I leave, I will lose some strength, and even leaving in the middle of the reserves is a bit disrespectful. On the other hand, my studies (a scientific field) are also important to the country, and knowing that I will return to the reserves again later in life, would it be right to concentrate on my studies? Or stay until the end of the reserves, even at the cost of damaging my course grades/postponing them to next year and disrupting the next school year?
Thank you very much.