What is the ruling on writing on the pages of the Mushaf, whether it be ordinary words or Tajweed rulings, with a pencil or ink, and for educational purposes or otherwise?

It is out of reverence for the sacred ordinances of Allah to protect the Qur’an from having non-Qur’anic text written alongside it. This is especially true if the text is extraneous and unrelated to it, in which case it is impermissible to write it on its margins. However, if it is related, such as an exegesis, a legal ruling, or a Tajweed rule that aids in learning and memorization, then there is no harm in it, provided that it is on the margin and written with a pencil for ease of removal. It is best to remove it when it is no longer needed.