Replacement?

Karyn entered into the class and sighed in relief that teacher wasn't here yet. She took her sit and wondered what was happening, Mr Matheson was always punctual. Other students also wondered what was going on. After 5 minutes a principal entered and Karyn knew that something must had happened. "Mr. Matheson couldn't arrived today for some personal reasons. Luckily we have a new teacher in school who will act as a replacement to Mr.Matheson in math classes for now." He pointed at the door.