The incident prompted a precautionary lockdown at Tennyson as well as nearby Glassbrook Elementary School.
At 8:40 a.m., police received a report of two people, one armed with a rifle, near the high school at 27035 Whitman St., Hayward police Sgt. Steve Brown said.
As officers arrived, both suspects ran away. One man, 20-year-old Juan Guzman, was arrested near the intersection of Mocine Avenue and Goodwin Street, Brown said.
The other suspect, who was carrying the rifle, was last seen running south in a culvert east of Glassbrook Elementary, Brown said. The suspect was not found, but police recovered clothing and the rifle, which was determined to be a BB gun, he said.
The lockdowns were lifted at both Tennyson and Glassbrook after it was determined that students were no longer in danger, according to Brown.
The suspect who is still at large is described as a Hispanic man about 20 years old, 5 feet 7 inches tall. He was last seen wearing all black clothing, including a black hat, Brown said.
Guzman, a Hayward resident, was arrested on suspicion of being on school grounds without permission, according to Brown.
The suspect carrying the BB gun allegedly pointed it at a passing bicyclist, and police are trying to locate the bicyclist as a witness in the case, Brown said.