Every year during the spring of their junior year, some of the next year's senior class have their Senior Pictures taken for a variety of different reasons. Perhaps the most common is that they want the cooler temperatures, and fresh blooms, of spring. They know their look won't change that much when compared to waiting a couple of months for their "Senior Summer" to begin. That's right, as soon as last year's senior class leaves the building, the senior class of 2026 will take over!
Another common reason soon-to-be seniors are photographed in the spring of their junior year is because they know they have a very busy summer ahead of them. Maybe it's a vacation, or several, or a band camp or basketball camp, some seniors are just busier in the summer, despite the fact they will be even busier when school starts, with all the classes, studying, and extra-curriculars.
Don't worry if you want to delay for summer, but beware of the hotter temperatures and higher levels of humidity. Spring is gorgeous for photography.