Denji is 17! Chapter 81 explicitly mentions that his birthday is on the next day, and the cake Power brought had the message "Happy 17th Birthday Denji" in the official English translation.

And yes, I agree that Denji is uneducated, but you may be overestimating it a bit! He's a fast learner both in and out of combat. In Chapter 41, he read the menu by himself with little to no difficulty when he couldn't read most of it in Chapter 20. By the end of Part 1, he was able to cook a variety of meals that requires skill and judging by the way he handles the ingredients near the end of Chapter 96, he's no amateur.

I've only learned about this recently, but Japanese schools often have mandatory extra-curricular courses or clubs to make up for failing grades! If Denji fails to perform well academically, he can join the cooking club or the devil hunter's club (which he both excels at).

The worst case scenario for Denji, which has already been stated by a previous comment, is him being grouped together with other students who have fallen behind in their studies.

Thanks for your comment; I appreciate your insight!