So my old high school had an assembly towards the end of the year when all the seniors would be dismissed from the gymnasium and they would head outside for the senior picnic, marking the end of their time in high school. Then, the principal said “Juniors, someone needs to fill in the senior spot. Go do so.” And all the juniors would run into the senior section and officially be seniors while the sophomores and freshmen head into the next section. That was when they were officially considered the next grade. At my new school, however, there is no official point where you’re a senior. I was thinking it could be one of three times. One may be when all the seniors leave and the juniors are the big guys at school. Another may be when finals are done and summer is on. Another may be when school starts. What’s your opinion on when a junior officially becomes a senior?
The day school starts.
From the time you leave school for the last time your junior year until you set foot on campus the beginning of the next academic year, you are a rising senior. Prior to the end of the previous school year, even if the seniors have been released, you are a junior.
While that’s the technical definition, it’s not really worth splitting hairs. 
The way most universities define things, when they ask for your grades up to the end of 11th grade, they include any summer classes you may have taken, say at a community college. So hence you are considered an 11th grader until the start of your 12th grade in August or September. So it can be an important distinction.