After we had our DD8 tested using the WISC-IV we also had her do the Woodcock-Johnson III achievement test. We called an I&RS meeting with key players at the school. Given the combination of IQ and achievement way in excess of her grade level they paid attention.
We had one teacher who had not thought our DD8 was that outside of normal have the integrity to admit that having seen the test results she understood why our DD8 wouldn't have been that motivated to do a few extra work sheets!
Regarding 'other kids being just as clever' I doubt that - unless you live in an exceptional place or your school is exceptional or the class size is huge I just don't see this as being statistically likely - it is still possible (obviously) but I do not see a teacher who doesn't even know what Pi is as being intellectually capable of appreciating just how rare an FSIQ of 151 is.
I think that you may need to escalate because your DS's current teacher is clearly not creating an environment that is fostering his intellectual growth.
