A response was posted by the group on social media.
In the letter, which was sent to parents on Wednesday 26 February, the local police force admitted the Youth Strike 4 Climate demonstration could bring large parts of Bristol to a standstill.
It warned parents and supervisors that "trips, slips, falls and crushing" could be seen at tomorrow's demonstration - which might see crowds reach the tens of thousands.
The teenage climate change activist and Nobel Peace Prize nominee Greta Thunberg will give a speech on College Green at around 11am on Friday.
She'll then join a protest march through the city, which will see Park Street and a section of the centre closed.
Thousands of young people are predicted to travel to Bristol from all over the country.
Environmental coach company, Snap, is offering bus travel to those who want to see Greta - and has had interest from around 1,000 people.
The company only runs trips if it sees enough demand for spaces, in a bid to get empty coaches off UK roads.
Bristol's bus service has also issued a warning ahead of tomorrow's event.
Ian Coyle, Operations Director for Bristol at First West of England, said:
We are working closely with local authorities to minimise disruption to our services on Friday. Delays are to be expected and diversions will be in operation for some services travelling through Bristol, particularly routes through City Centre. We encourage all customers to factor in more time to their journeys and check the First West of England website and social media channels for updates, as diversions may change throughout the day. The safety and wellbeing of our customers, drivers and the public is our number one priority and we urge all those using our services to remain patient, whilst we work to continue to operate our inner-city routes as best as possible.