Hi everyone. I am doing an analysis of the effects of access to school-based health centers (SBHCs) on algebra achievement. I have collected data on school districts in Maryland that implemented an SBHC between 2007 and 2016, as well as control schools that never implemented an SBHC. I am using a difference-in-difference technique to examine algebra scores pre- and post-implementation of SBHCs, and I am having trouble with my regression because each school district implemented their SBHC in different years. I have created dummy variables for each year relative to a school's implementation of an SBHC -- for example, if an SBHC was implemented in 2010, then that district has a 1 for dummy variables one through six, corresponding to 2011 (the first year following implementation) through 2016, as well as 1 for dummy variables -1 through -3, corresponding to the pre-treatment years of 2007-2009. My main question is -- should each of these dummy variables (I have seventeen of them ranging from -8 to +9) be included in the regression along with my fixed effects for year? This doesn't seem right to me, but it was suggested to me by my professor, unless I understood her wrong. Alternatively, I have created one single variable, called time, which contains this same information with values ranging from -8 to +9. However, both of these tactics seem problematic because they only pertain to school districts that implemented an SBHC at some point and ignore those that never implemented an SBHC.

Here is what I've written so far:

Code:
reg basicpct treat i.school_id i.year negeight negseven negsix negfive negfour negthree ///
    negtwo negone one two three four five six seven eight nine, robust cluster(school_id)
basicpct is the percent of students achieving basic proficiency in Algebra in a school district, and treat is a dummy for school's treatment status.

I feel really off-base, and I'm pretty sure I'm in over my head. I would really appreciate it if anyone who is more experienced with difference-in-differences could walk me through this. Thank you so much in advance.