Is it preferable to perform the Istikhara prayer before studying and booking an exam, or is a Muslim always obligated to seek knowledge?

Seeking worldly knowledge is permissible and may become recommended with a good intention; one may perform Istikhara (prayer for guidance) regarding it. As for religious knowledge, it is desirable, and Istikhara is prescribed for it when there is a conflict between two desirable matters. Otherwise, the prescribed action is to proceed without Istikhara.