Built a free practice site for Romania's national exams. How do I market it efficiently without spending too much on ads?

Hi! This summer I started a solo project: a free practice site for the Romanian national math exams (the baccalaureate and the 8th-grade exam). I built it because I couldn't find a good place to practice on the official past papers and check my answers right away. The feedback so far has been really positive. In it's first week it had about 650 visitors and it now has 70 active accounts. What I'm struggling with is marketing. I don't really understand how to run ads that actually work, or whether paying for ads even makes sense at this stage. Most of my users are high school students, so targeting is limited too. Do you have any guides you'd recommend, or tips for getting better at marketing a product like this? Thanks!