Penguins Swim School, based at Lancing College, raised more than £1,000 for long-term partner Shoreham RNLI as members celebrated the landmark anniversary.
Founder Linette Wheeler set up the school in 1994 with her sister Gill Button, who is also a Penguins teacher.
The sisters, who grew up in Sussex, were inspired to start swimming careers by their father, a competitive swimmer and county water polo player, and their mother, who supported them through hours of training and competing at Sussex county and national level.
Linette said: “It has been an immense privilege to watch so many generations of swimmers gain skills and confidence in the water and achieve more than they believed was possible.
“Every single swimmer who has come to Penguins in the last quarter of a century has been nurtured and supported as an individual - and that would not have been possible without our amazing team of devoted teachers.
“As we look back on the last 25 years, we are celebrating the hard work, the commitment, the warmth and passion of all our teachers and swimmers.”
The founders said it has been particularly rewarding to see swimmers who began as Little Penguins aged four or five grow up to join the team as swimming teachers themselves.
Other highlights of the last 25 years include being named UK Swim School of the Year in 2016 and Southern Water Swim School of the Year in 2018.
Penguins marked its anniversary with a swimming gala, where swimmers took part in fun events. The gala raised more than £1,000 for Shoreham RNLI, a long-term partner of Penguins.
Linette said: “We are very proud of our long-standing relationship with the RNLI, which provides an essential community service, and reminds us that swimming is not just fun – it is a vital life-saving skill.”
Penguins is now looking ahead to the summer, when it will host five days of beach safety activities on Lancing Beach Green from July 24 to 28, as part of the nationwide Swim Safe scheme.
Bookings can be made from six weeks beforehand at www.swimsafe.org.uk. Last year, this free event was attended by more than 300 children.