The OP is already answered halacha lemaase by NJM citing great poskim. I want just to try to find some sources in Shass. My approach is to help understand (partially) halacha only. For this we need a quick review of Chazal.
There are three issues in halachic terminology:
- Daber Davar,
- Oneg Shabbat.
Apparently the prohibition of preparing (Hachana) is addressed in Gemara (Eruvin 38b Kushia of Abaye, see there in Chidushei Harashba) for a physical effort, e..g. walking a great distance to make an eruv. Regarding speaking the prohibition addressed is "Daber Davar". In Mishna Shabbat (see Talmud Bavli 113a) there are examples of technical preparation for bed-making, vessels washing and folding clothes. For thinking or reading alone no prohibition I didn't find sources calling them Hachana (contrarily, the Gemara allows: "hirhur mutar", Shabbat 113b).
To read secular matter in Shabbat is discussed e. g. Medicine or astronomy and is an independent issue.
At first glance if somewhat is permitted to study "per se" the day of Shabbat, no problem if the intent is to achieve a test. If the matter itself is not allowed on Shabbat, obviously we cannot learn it, even without test issue.
Studying, if the intent is the test only, may perhaps may perhaps be prohibited because of "Daber Davar", your speaking of Shabbat needs to be different than your speaking of Yom Chol. If he learns by speaking and not thinking only, and speak alone, it is not a "performative speaking" but rather a thinking equivalent, it's not relevant in matter of Dabber Davar. But we know that even thinking is not good because ofthe cancellation of the positive mitsva of "Oneg Shabbat".
An additional reason to allow secular activities is the "mitsvatic"content associated with them. E.g. asking for own lost object. If there is a mitsva in the speaking with others e. g. in way to generate mitsva of hashavat aveda, it is allowed. We can imagine a mitsva for at student in diverse ways.
This answer is not halachically relevant because some poskim cited in the answer of NJM prohibited due to hachana.