I am trying to evaluate a government program where poor schools were selected and given money to improve their infrastructure. I am using results of standardized tests as the outcome variable, and I have a dummy for whether the school received the treatment (aid from government) or not, and I want to compare 2013 results (before treatment) with 2015 results (after treatment)

First, I matched schools considered for aid with their 2013 results, eliminated those that did not match, and created a variable called dummy2013 with a value of one for all.

Second, I matched schools considered for aid with their 2015 results, eliminated those that did not match, and created a variable called dummy2015 with a value of one for all.

Note that most schools matched for both years, but some do not.

Finally, I used the append command to combine the databases and created a dummy called after_dummy, with a value of 1 when dummy2015 was equal to 1 and 0 when dummy2013 was equal to 1. I created the dummy for the interaction and then I run the regression, but stata dropped both the after_dummy variable and the interaction term due to collinearity, what I am doing wrong? How can I solve this?

Thanks a lot in advance.