The Linden School looked quite ordinary that morning. Children were running around in the corridors, laughter echoed, and the caretaker Mr. Tomasz was watering the flowers as usual. But that day, when Igor, Maja, Wojtek and Lena came to class, they immediately noticed something strange. It all started during the break after maths. Suddenly, at exactly 10:10 a.m., instead of the usual loud bell, there was a different, mysterious sound - quiet, melodious, as if someone was playing an old music box. The pupils exchanged puzzled glances, but the teacher, Mrs Ela, reassured them by explaining that it was probably a temporary malfunction. However, Igor noticed that in the corner of the corridor, under the old notice board, something had appeared that wasn't there before - small, shiny buttons in different colours. Each had a different design: one was star-shaped, another heart-shaped, the third resembled a key. "Hey, look! This wasn't here yesterday!" - whispered Igor to his friends. All four quickly walked closer. On the blackboard was written in silver letters: "Open the right button and you will discover the secret of the school". Maja trembled with excitement. "Do you think this is some kind of school game? Maybe Mrs Ela has prepared something?" - she asked. Wojtek held out his hand towards the key-shaped button. "What if it's not a game? Maybe someone wants to tell us something?" - he said in a whisper. At the same moment that he touched the button, the wall next to the blackboard began to tremble, and just below it emerged a narrow door that no one had known about before. The door was slightly ajar, and a bluish light was seeping from behind it. The friends looked at each other in disbelief and delight. Igor's heart was beating hard. What would they find on the other side of the door? Should they go in, or should they run away after all and notify the adults? Suddenly, a quiet whisper came from behind the door, inviting them inside: "It's time to learn the secret of the Linden School..."