Learn Dutch: De kinderen zijn in de klas. - The children are in the classroom.

You use in because the children are physically inside an enclosed space. We use op for surfaces or open spaces (like on a table or on the street). Since klas here refers to the physical room, in de klas is the correct choice.

Does klas mean class or classroom?

It can mean both! Depending on the context, de klas means either the group of students or the physical room itself. If you want to be very specific about the room, you could use the word het klaslokaal, but just saying de klas is perfectly normal for everyday speech.

Why is the plural of kind spelled as kinderen?

Most Dutch words become plural by adding -en or -s. However, kind is an irregular noun. It adds an extra -er before the -en to become kinderen. It is very similar to how the English word child changes to children.

Why does the verb zijn look exactly like the dictionary form of the verb?

In Dutch, the plural form of a verb (used for we, you all, and they) is almost always exactly the same as the infinitive (the dictionary form). Since de kinderen is a plural subject meaning they, we use the full form zijn.