Learn Polish: Nauczyciel jest w szkole. - The teacher is at school.

In Polish, w is used for enclosed spaces and buildings. Even though we say "at school" in English, Polish treats the school as a physical building you are inside. You literally say "The teacher is in the school" (w szkole). You generally use na for open spaces, events, or certain institutions.

Why does the word szkoła change to szkole here?

Because we are expressing a location! After the preposition w (meaning "in" or "at"), we must use the Locative case to answer the question "where?". This means the dictionary form szkoła changes its ending to match the grammar rule.

Why does the letter ł change to an l in szkole?

For feminine nouns ending in -a like szkoła, the Locative case ending is -e. When we drop the -a and add this -e, the consonant before it often "softens" to make it easier to pronounce. That is why the hard ł changes to a soft l, giving us szkole.

Does nauczyciel mean a male or female teacher?