I have read in Fiqh Sunnah that I can join together Thuhr and Asr pray and Maghrib and Ishaa. And also that I can pray sitting on the chair. Time to time I have problem with praying at school, that I cannot pray normally in the right time. What is better, to join prayers or pray on time sitting?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Combining Thuhr and 'Asr, or Maghrib and 'Ishaa' is permissible in particular cases when travelling or when it is raining and in other circumstances that are mentioned in the books of Fiqh (Islamic jurisprudence). Studies are not a sound religious excuse to combine two prayers while being a resident. Therefore, it is an obligation upon you to perform the prayers at their fixed times. Allaah says (which means): {Verily, the prayer is enjoined on the believers at fixed hours.}[4:103]. This means that they have to be performed at a particular time. Praying an obligatory prayer while sitting is only permissible when the person is unable to perform it while standing, as the Prophet said: "Pray standing, however if you are unable, then pray sitting, and if again you are unable, pray lying down." [Al-Bukhari] So, if a person is in sound health and able to stand, then it is an obligation on him to pray standing, because standing in prayer is one of the pillars of the prayer. As regards praying sitting down on a chair at school because of studies then this prayer is not acceptable and whoever prayed in this manner has to make it up.  

Allaah knows best.