Learn Dutch: De leerling neemt deel aan het toernooi. - The pupil takes part in the tournament.

They belong to the separable verb deelnemen (to participate/take part). In a standard main clause like this one, the verb splits: the conjugated part (neemt) takes the second position in the sentence, and the prefix (deel) moves toward the end.

English says "takes part in", so why does this sentence use aan instead of in?

The verb deelnemen uses a fixed preposition. In Dutch, you always say deelnemen aan (literally "take part at/on"). Prepositions often don't translate directly between English and Dutch, so it is best to memorize deelnemen aan as a set phrase.

Could I put deel at the very end and say "De leerling neemt aan het toernooi deel"?

Yes, you can! Both word orders are perfectly fine and mean the exact same thing. While the prefix of a separable verb often goes at the very end of the sentence, prepositional phrases (like aan het toernooi) are allowed to be placed after the prefix.

This lesson is about focus adverbs. How would I change this sentence to say "Even the pupil takes part..."?

You would put zelfs (even) right at the beginning, directly in front of the subject: Zelfs de leerling neemt deel aan het toernooi. Focus adverbs like zelfs and vooral usually sit immediately before the specific word or phrase they are highlighting.