Is a sleeveless vest warm enough for school trip?

[NaiceBee]

My 14 year old daughter is going on a school trip and insists on wearing a relatively thick sleeveless puffer vest over a short sleeved t-shirt. It will be around 10°C with only 4°C in the morning. She will be mostly outside. In my opinion that'll be way too cold but she doesn't want to hear that. What do you think?

Well, most 14 year olds think they know everything. I would pack a warm sweater in the bottom of her bag. Useful in an emergency in case someone needs to be kept warm. If no emergency happens and she changes her mind she can use it and take the credit for planning ahead.