Senior Night is the Friday night at the football game when the seniors on the football team and in the Marching Band are honored by having their pictures taken with their parents and presenting their mothers with roses and then forcing them to stand out in front of everyone so the audience can clap and yell "Hooray!" and remember how many years it's been since they were Seniors, on the cusp of the "real world", ready to leave High School and move on to bigger and better things.
It is also the night when the Seniors get a chance to look back on the year so far and realize that it won't be very long before all this fun and games with the instrument playing and marching and wearing slick uniforms and going to competitions is going to come to a dramatic end because they're going to have to come up with a real career and a real life that won't involve dressing up in fancy outfits every Friday evening and going to a stadium full of screaming football fans and trying to avoid being run over by three hundred pound fullbacks who don't care that it's supposed to be the band's turn out on the field.
It's also the night when little sisters (although not quite so little anymore) realize that it won't be very long before her big brother will be moving on and moving out, getting a life and getting a career that will not involve dressing up in a fancy uniform and marching around a little faux-grass field, while she still has two more years of this very interesting activity to go.
No wonder she doesn't look pleased.