To the issue of size, I have seen a lot of variance in trees planted at the same time here. I have a shelterbelt of Picea pungens (Colorado Spruce). They were planted in 1995 from PFRA (Prairie Farm Rehabilitation Administration). They were tiny, less than 10" in height. The average size now is 10-12', but I have a couple that are barely 2 feet!
There is a soil anomaly where one lil one is, and as they are planted from the base of a hill to the top, water is a contributing factor to their size. The trees at this particular school are more than likely to be the same age.
Burnham, yes sunscald (or sunburn) is a consideration. So is wind evaporation on the bare soil that is usually found under evergreens here. Evergreens drop needles which in turn alter the pH of the soil turning is slightly acidic. This is believed to be their allopathic remedy to competition of other plants and weeds. Evaporation of moisture from wind seems to be huge. I have seen trees competing with grass and hedges, but that type of competition is small compared to the loss of moisture through wind evaporation. That is why I insisted that the CRZ be mulched if I raised the crown at all.
Like I said previously, the crown raise will only be high enough to get a push mower close. I leave my spruce trees touching the ground, kinda like letting the plant (tree) determine how it wants to grow. The School official did agree with me that his staff was lazy!