If it's 25 metres in your swimming trunks and no fins, then you'll need to practice, especially if there's no kicking the wall.
If you can use dive kit then must you use flappy finning or can you use something far more efficient such as frog kick?
As you swim, you'll get the urge to breathe. Resist it and it'll easily give you the extra seconds you need to get to the end.
Sorry, but frog kicking is far LESS efficient for covering a distance holding your breath...
If you see videos of long distance records, all employ proper flutter or dolphin kicking, The most efficient is dolphin kicking, possibly with a monofin, but very few divers master this technique. It is usually employed by athletes of fin swimming, who switch from speed to distance competitions.
Flutter kicking is just slightly less efficient, and most free divers manage to learn it decently, and use proper long free diving fins. Frog kicking is good for scuba diving in tight spaces, where you want precise motion control and no silting. But it does not allow to cover easily long distances.
If no fins are allowed, then underwater breast stroke is the way. In this case, most of the propulsion comes from arms (as frog kicking is even more inefficient, without fins). If you have bad frog kick, then probably it is better not use the legs at all, or to use them is some sort of dolphin kicking, during the recover of arms (when you bring back the arm from your sides, after the stroke, towards you, for preparing for another stroke).
The underwater breast stroke is also called the "alternate stroke"; as there are long pauses, after each stroke, where you just stand still, running by inertia. Very good underwater swimmers cover one pool of 25 m with just TWO strokes. Never arrived at that, but I usually make 5 full strokes (that is, 5 arm strokes and 5 leg frog kicks). In some rare cases I managed to make the pool with just 4.5 strokes, and once in just 4 strokes (but I was much younger).
Last trick is proper ventilation before the apnoea. I am not talking about hyperventilation, which is a dangerous practice, easily bringing you into a hypoxic state, without CO2 increasing enough for warning you. But a simple, calm and profound ventilation for one minute is a safe practice, which will extend your freediving time by at least 50%.
