In a survey that I'm running I ask someone what age they started school and what age they finished school.

I currently ask them how many years they have been in school, but I'm wondering if I can automatically caluclate that using the two numerical responses for the age started and age finished school?

I'm asking the question in an array with a age started, age finish and number of years in school as three questions on the y-scale.

Is this something I could do with the expression manager?

I've attached the .lss file with only this question I am working on.

This is definitely something for Expression Manager. Have a look at the example of the BMI calculator. I am sure you can use parts of it for what you need (which is much simpler than the BMI calculator).