An increase in the school leaving age would mean a decrease in the participation rate and increase in unemployment right?
Yes, in this situation, you would see a decrease in the participation rate (those who participate in the workforce, both employed and unemployed but seeking work). This is because students are not counted in the unemployment statistics, as they do not fit the definition of unemployed, which is "unemployed but is actively seeking work". Therefore, they are not counted as participating in the workforce.
However, it would increase the unemployment rate, as the labour force participation rate decreases. Therefore, when using the formula to calculate unemployment, you would see an increase in the unemployment rate.
Hope that helps =)