Swimmers queuing outside troubled Hinksey Pool in Oxford were denied admission -- because some lifeguards were sitting exams.
Most lifeguards who work at the outdoor pool, off Abingdon Road, attend Oxford Brookes University or Oxford University.
On Monday, their exams clashed with lifeguard duties.
Safety regulations require eight lifeguards to watch 250 bathers, the maximum number allowed into the pool, and when only five lifeguards were able to work, swimming numbers were cut to 150.
The restriction meant frustration for swimmers who queued up to get into the pool, which is open from May to September.
Yesterday, pool manager Lucy Cherry said: "Monday was our busiest weekday session since the start of season, but we had no queues until 4pm and this lasted until around 5.30pm.
"All customers in the queue were regularly updated and told the wait would be approximately 30 minutes.
"We also inform people that as lifeguard's shifts finish, we need to reduce the numbers of swimmers and there will be times when the one in-one out policy doesn't operate."
The majority of lifeguards are students because it is difficult to recruit local people to fill the summer role.
Last month, work to repair damaged tiles closed the pool for five days.
Oxford City Council said the damage was caused by a combination of changing water temperatures and frost damage over the past six years.
The week-long closure at Hinksey Pool followed the closure of Banbury's outdoor pool, after young swimmers complained of abrasions to their feet.
Sore feet also led to the closure of Abingdon's Abbey Meadow pool two weeks ago.
And Carterton's open-air pool could close in September, because the town council faces a £100,000 repair bill.
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