Not sure who you spoke with at HMRC, but they should have told you about the Simplified PAYE scheme. The Lower Earnings Limit for NICs for 2007/08 tax year is £87.
Not sure if the being a student has changed things, perhaps, but better to complete the paperwork than risk HMRC changing their mind in the future.
According to P16, tax deduction for £100 weekly pay is £17.03 under the simplified scheme.
Under the Simplified Scheme, NICS are only paid when earnings exceed £100 weekly, however NICs liability starts at the Lower Earnings Limit, £87. Therefore you should complete a P12, so that your employee does not lose out in the future due to incorrect NICs. Also P12 should be completed as there is £17.03 a week in tax payment due.
You should always give a payslip, even if it is just stating the amount paid, plus any deductions from GROSS salary - even if they are Zero.
I would suggest calling the HMRC New Employers helpline again and asking about the Simplified PAYE Scheme, explaining that you will be paying £100 per week, which is above the Lower Earning Limit, but is currently the Earnings Threshold.