When you try to connect from your school, does the server console show anything relevant?
Perhaps try forwarding an additional, different port than the default minecraft. Usually home NAT devices allow you to port-forward WAN port X to LAN port Y, where X is different to Y. Some ports will most likely be blocked by your ISP, but perhaps some will work. Try to set up an additional forwarding from port 80 (WAN), from port 443 and from port 1025, each to port 25565 (LAN). Let your remote friend verify that those ports work for them, then try again from school.
Perhaps try forwarding an additional, different port than the default minecraft. Usually home NAT devices allow you to port-forward WAN port X to LAN port Y, where X is different to Y. Some ports will most likely be blocked by your ISP, but perhaps some will work. Try to set up an additional forwarding from port 80 (WAN), from port 443 and from port 1025, each to port 25565 (LAN). Let your remote friend verify that those ports work for them, then try again from school.