VS Redirection while maintaining Source IP

You should do a Redirect based on your compare, since the Client IP Address (Schoo's IP Address) will remain the same after the redirect.

After the redirect they will persist to the Webmail Virtual Server using the settings applied to that Virtual Server.

when HTTP_REQUEST {
 IP::client_addr explained at http://devcentral.f5.com/Wiki/default.aspx/iRules.IP__client_addr
if { [IP::addr [IP::client_addr] equals xxx.xxx.xxx.xxx] } {
HTTP::redirect "https://webmail.domain.com"
}
}

If you decide to do some type of validation on the Webmail VIP, based on the Client IP Address coming from a specific set of Subnets, you can base that iRule on this one an modifiy the compare. See this as a reference:

http://devcentral.f5.com/Wiki/default.aspx/iRules.IP__addr