Prevention in School (PS) is a modified School-Wide Positive Behavior Support Program at the universal prevention level, developed by Swedish researchers for the school years 4 -9. By defining and teaching school rules to all teachers and students, and by consistently acknowledging students when meeting behavioural expectations, the program aims to reduce problem behaviours in school areas and to improve the classroom learning climate. Against this background, the purpose of this study is to examine whether PS is more effective than schools' ordinary practices in meeting these goals.

Inclusion Criteria:

• Schools situated in Stockholm county and in bordering counties within travelling distance from Stockholm.
• Comprising the grades 4-9 (10-15 years).
• A minimum of 2 classes/50 students in each of the grades 5-7.
• A majority (min.80%)of the staff is positive towards program participation.
• School management willing to take active part in change/developmental work.
• A willingness to submit to study conditions (randomization etc)

Exclusion Criteria:

• Participation in other ongoing research projects.