Learn Russian: Школьник идёт в школу.

When you talk about going to a destination, Russian uses the Accusative case to show direction. The dictionary form of the word is школа (a feminine word ending in -а). In the Accusative case, that -а changes to -у, making it в школу.

Why does the sentence use идёт for "is going" instead of ходит?

Both words mean "to go" on foot, but идёт is used for a one-way trip happening right now. It means the schoolboy is currently on his way to school. If you wanted to say he goes to school every day as a habit, you would use ходит.

Could I say идёт в школе?

No, because в школе uses the Prepositional case, which talks about static location. If you said идёт в школе, it would mean he is already inside the school building walking around. To show motion toward a destination, you must use the Accusative case в школу.

Are the words школьник and школу related?

Yes! Школьник means "schoolboy" and is built directly from the base noun школа (school). If you wanted to talk about a schoolgirl, you would use the word школьница.