Officials confirmed one person is in custody, and a weapon was recovered.
Just before 1:30 p.m., shots were reported near 13 Mile Road that sent those on the island for spring break scrambling over dunes, causing chaos.
Officials said many tried to climb or drive over the dunes, which serve as a barrier between the beach and the island's homes.
Police said many who ran were participating in an unsanctioned "Senior Skip Day."
Galveston PD posted on social media earlier in the day warning drivers of a heavy law enforcement presence and to expect long delays as many students were descending west to the island for the event.
Galveston Police Chief Doug Balli said no one was injured.
Just before 6 p.m., officials confirmed that Hershey Beach, 11 Mile Road Beach, and Sunny Beach at 8 Mile Road to 13 Mile Road Beach had reopened after being closed due to the investigation.