# What statistical test when the dependent variable is the difference between 2 means?

Children born in the summer perform weaker in school tests than autumn born children (mostly due to them being younger in their school year/grade). I have the test data for a class of 30 children and I have found the mean grade in science, reading, writing, and maths for the autumn and the summer born children in the class. I have subtracted the mean autumn grades from the mean summer grades for each subject, giving 4 'Gap Sizes' (my dependent variable).

My first question is: what statistical test could I use to test if there is a significant difference in the season gap size between the 4 subjects? I also have the data for the grades of the children from their first year of school. My second question is: How could I, for example, test to see if the gap size in science in the first year/grade of school is significantly different from their current year?

The fact that my dependent variable is simply the difference between 2 means (1 for each subject) is what has confused me.

Any help would be much appreciated.