FYI, here are the lines I had to add to the iPxe config to deal with the other TFTP image server on my school network:
#already had this line in place
set next-server 10.xxx.xxx.xxx
#additional lines to make sure correct boot image and image server are used
set filename pxelinux.0
set proxydhcp/filename pxelinux.0
set proxydhcp/next-server 10.xxx.xxx.xxx