number of obs in mixed effect model, panel data

Hi,

I have a data set of 4,500 children's' weight status and their lifestyle behaviors and two waves (baseline and follow-up) are included. Now I am trying to examine the relationship between weight status and the lifestyle behaviors, since these children are from different schools, I choose to use the mixed effect model. However, after I reshape my data to long format and run my command(below), I get the results: Number of obs = 9,000. I am very new to touch the panel data and wonder if I choose the right command for my aim above. I will appreciate very much if you can give me any suggestion.

my command: xtmixed bmiz sleep screen hw i.fastf i.ssb fruit veg year ||school: year,mle nolog cov(unstructure)

Thanks and happy holidays!!

Chris