Judges are provided with 'starting points' and 'ranges' which they must normally adhere to when sentencing offenders. (If they go outside of the specified ranges they must give an explanation in open court).
The starting point for a first offender convicted of a 'Section 20' offence (where there was no pre-meditation, no weapon used and no grave injury sustained) is 24 weeks custody. The upper end of the sentencing range is 36 weeks custody. The bottom of the sentencing range is a 'high level' community order.
However those sentences apply to first time offenders
convicted after a trial. An early guilty plea will skew the sentencing downwards, giving a starting point of around 16 weeks custody but (more importantly) increasing the chances of a non-custodial sentence.
The courts generally try to keep people out of prison wherever possible, so your character references (and the impact that a custodial sentence would have upon your education) may well be enough to prevent you being 'sent down'.
Plead guilty, look remorseful and let your barrister do the talking.
Source:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(See the table on page 15).
Chris