Hi Urpi,
The first place to go are the handbooks for teachers. Think twice if you want your students to take regular PET/Preliminary (fine if you have adult students as there are questions about jobs, career etc.) or PET/Preliminary for Schools (more suitable vocabulary and themewise for teenagers and people who are at school). Just in case, here are the links to both:
www.teachers.cambridgeesol.org/ts/exams/generalenglish/pet
www.teachers.cambridgeesol.org/ts/exams/younglearnersandforschools/petforschools
There are also vocabulary lists there. Hope this is something to start with.
In the certificate it doesn �t say if a candidate took Pet or Pet for Schools, so they shouldn �t worry :-)