The Seagulls drew 1-1 at Leyton Orient and although they lost the subsequent penalty shoot-out 4-2 they topped Group C after earlier wins over AFC Wimbledon and Southend.
Rusk said: “We had ten under-18s in the squad, so I’m absolutely delighted. That’s what the tournament is about for us and it’s a great challenge.
“With such a young team I thought we were outstanding in the first half. We moved the ball well, looked threatening on the break and scored a good goal then had another perfectly good goal disallowed which wasn’t offside.
“In the second half their experience and physicality showed and there were moments when we had to defend our box well, but we have won the group and to remain undefeated is a really good achievement for the squad.”
Danny Cashman followed his goal in last Friday’s Premier League 2 win over Southampton with another as Albion led at the break before being pegged back in the second half by Louis Dennis’s equaliser.
“Danny was bright at Southampton and he took his goal well tonight so that will be good for his confidence,” added Rusk.