Am I still vulnerable to attack?

[cdunn () foxwood notts sch uk]

Hi,

I work in a school in nottingham, UK we access the internet via the EMBC who inter-connect all schools and colleges to 
the internet. The EMBC manage our site blocking and spam filters for E-Mail. The proxy we use to access the net is 
heavily filtered and all un-used ports are closed, making an attack as far as I can see VERY difficult.

My question is just how vulnerable is our school network? is there still potential there for an attack to take place? 
or would this be so difficult and the rewards to few for an attacker to bother?

Thanks

Craig Dunn