Hi August,
Yes, it is very rare to have that kind of scatter - I'm not sure of the exact likely hood, but it is very very rare. My son has close to a 50 point spread between his VCI and PSI - and coding is also one of he's lowest scores. Your principal is right that it is not uncommon for a 2-e student to have a similar pattern of higher VCI &PRI scores compared to PSI. But it is not a typical pattern by any means.
Did you have him tested in hopes of getting acquiring accommodations? What issues is he having? Were there other tests? What were the index scores? Does he have any OT/motor issues? How about concentration/attention?
Generally, decisions about accommodations are based on a battery of tests, observation, teacher input, work samples and history. Your school should not rule in or out based on a WISC score alone. However, with such wide scatter, they SHOULD dig deeper and try to learn more about your child's learning profile and to rule in or out disabilities.
I anything more than one standard deviation is considered "statistically significant". The WISC subtests have a mean of 10 and standard deviation of 3. This article may prove helpful
http://www.wrightslaw.com/advoc/articles/tests_measurements.html#10
