I've had diabetic children in my class before. I've done injections for one, and programmed an insulin pump for another. Could this child be offered a pump if they are needle phobic? Not your decision I know, but something to query. Treatment has moved on so much now. I'm surprised a needle phobic child isn't being offered a pump.
It is a huge responsibility, but a necessary one to take on as a duty of care for a child in your care. You will be given full training before you are signed off to administer independently. Training will include how to test blood, how to inject insulin (plus how to operate a pump, should this information be needed instead), how to manage hypos and hypers etc. At least 2 members of staff should be trained up, in case of staff absence or unavailability (best practice would be 3+). You will also need to be witnessed by another trained member whenever you administer a dose, to cover you and sign that they've witnessed. Again, best practice is to alternate who administers and who witnesses, so everyone can keep the skill up. Nobody wants to be in the position of having to administer one day, having not done it since the training months ago.
The part I would query is that you need to carb count. It is the parent's responsibility to provide this information if they have a packed lunch, or should be provided by the catering company if it's a school dinner (this information should be readily available from them). The child should then have a book which will tell you the dosage of insulin required, based on their carb intake and blood readings.
At the end of the day, if it's something you're truly not comfortable with you need to address it with your SLT, but I don't think it's an unreasonable ask that the class teacher is involved (if they are comfortable with it). What would be unreasonable (and unlawful) would be expecting you to take his on solo. Personally I would advise at least attending the training before reassessing how you feel. It may put your mind at ease. You definitely shouldn't be the only one trained though, not do I believe you should be carb counting.