Solution

24738

First name
Daniel Pearce
School
Cleeve
Country
Age
11

The person who wasn't in school today was Tessa.
My method was:
1. Put down a list of all the letters that people's first names started with.
2. Made a tally chart of all the people.
3. I checked my results with the information provided.
4. I found that I had got 3 people with T but the computer had 2 and they were boys.
5. That meant that the person who wasn't there had a name begining with T.
6. The only other person who had a name begining with T that wasn't 2 boys was Tessa.