Chords - steal em'!!!! Chords are not copyright, so you can steal the chords to the Beatles 'Yesterday' and put ur own tune to it. Just don't copy the counterpoint that they did with the chords, coz that is illegal! Either that or choose a simple structure.
Melody - here's the easy part. Write a two bar phrase. take this two bar phrase, and invert it for bars three and four. Take this 4-bar mini-melody and do a retrodgrade on it, turning it into an 8 bar melody. You can use augmentation and diminution as well, perhaps to turn it into a 16-bar phrase?
The first thing to think about is structure - set it out in ternary form or something, the important thing is to plan it first. Then, write a simple chord structure. This is 'A'. Modulate to a related key (Tonic Minor, Sub Dominant, Dominant) feel free to use an unrelated key if u feel adventurous!!
Get a chord structure for this new modulated bit, that you're 'B'. Put a tune over the whole thing and wallah! simple as that!
Do you have to score it or just write the chords and the melody, like on the keyboard and let the keyboard play the 'accompaniment'?
Try to keep it diatonic, you can't loose with that!
Instead of doing fancy modulations within a melody use things like unaccented passing notes, passing notes, auxilary notes, appogiatura's (not in the ornamental sence!), etc.
Not that you probably need it, here's a few descriptions of the words I've used in here:
Retrograde = Backwards
Inversion = upside down (ie: major fifth up, becomes a major fifth down)
Augmentation = make the notes longer (not nessesarily double)
Dimunition = make the notes shorter (not nessesarily half)
Try and stay away from straight arpeggio writing. If you're in a chord of C, don't just stick to notes C, E and G, use passing notes and stuff like I recommened above. It will make it sound less cheesy and will actually make it easier to get to the next chord.
Since you've only got 25mins to write it, don't write fancy chords - the rule of thumb in harmony is that you can only write music with chords 1, 4 and 5 (C, F and G, for example) for no more than 3 mins coz people get bored!!
use some nice touches of chord 6 and chord 3, though.
Finish off with a perfect cadence going from the dominant seventh to the tonic, you can even be ultra clever and do a perfect cadence utilising the Bdim chord (in the key of C) This gives your cadence a less cheesier feel, but just depends on what you want, really.
If you wanna be fancy you can add in a flattened 3 chord, in C major, this would be Eb major (although this involves modulation - but it's a great trick but needs some careful thought to use properly, still if you take 26mins to write your piece, I'm sure they won't mind!)
I've written more than I was mean't to, in fact wayyyyy too much!
But hope at least a bit of it was a little bit useful.
Good luck
