School children hold placards and shout slogans as they participate in a protest outside the Scottish Parliament on March 15, 2019 in Edinburgh, ScotlandCredit:Jeff J Mitchell/Getty Images Europe Thousands of young people, including those of primary school aged, marched in central London over climate changeCredit:Matthew Chattle/Barcroft Images Show more Thousands of school children descended on Westminster today for a second strike demanding the government change its environmental policies.More than 15,000 youngsters gathered outside the Houses of Parliament to call on politicians to declare a “climate emergency” to deal with the “ecological crisis” threatening the planet.They marched from Parliament square at midday to the gates of Downing Street, before congregating around Buckingham Palace where they were met with a heavy police presence.A few of the placards read: “By the time we’re in charge it will be too late”, “we want you to panic” and “the greatest threat to the planet is the belief someone else will save it”.Some of the other students formed a sitting protest on Trafalgar Square and climbed onto Nelson’s Column.The demonstrations caused traffic to be severely hampered along Westminster Bridge and Parliament Square, as well as Waterloo Bridge Roundabout.Youngsters have also today staged events in other British towns and cities including Edinburgh, Canterbury, Oxford and Cambridge. It began when 16-year-old Greta Thunberg, from Sweden, skipped class to sit outside government buildings in September, accusing her country of not following the Paris Climate Agreement.However, school leaders have warned of the dangers of pupils missing valuable lesson time and have voiced their opposition to the protests. Sueleen Deelan, an 18-year-old studying her a-levels at Mill Hill County High School, arrived at the demonstration late because she had an exam earlier in the morning.She said: “We need to take priority in conserving our environment and that’s why all the young children who are going to be the most affected by this are out today on a cold, windy Friday to show that young people are paying attention to current affairs and we care about the future.“I take Geography and Politics so my teachers were hugely supportive of my decision. I know from an academic perspective that there is a big problem and my school was quite proud of me for coming here because I was actively taking a role.”The action is part of a much wider global movement- spanning from Germany to Australia- known as Schools 4 Climate Action. Geoff Barton, General Secretary of the Association of School and College Leaders (ASCL), said it would be reasonable to expect students to make up any missed work during lunchtime detentions.Mr Barton said: “The more we say climate change is such an important issue that young people can go and protest about it, well the more you open up other issues.“What about fracking, what about homelessness for example, what about knife crime? It seems to me that we patronise children ultimately by saying ‘yes, well done’. It is a sentimental response.“They should be learning in school about why climate change matters and learning how political processes work.”Some headteachers have also expressed their suspicions about the march being used as a cloak by some students to play truant from their lessons.But, the protesters hailed the day as a success and pledged to come back every month “until the Government listens”. The Youth Strike 4 Climate in the UK follows action taken by 15,000 students last month, which was criticised by Theresa May for wasting lesson time and increasing teacher’s workloads.But, Anna Taylor, 17, from north London, co-founder of the UK Student Climate Network and one of the organisers of the Youth Strike 4 Climate movement, said the Government was failing to recognise the severity of the crisis. She said: “We’re here because we feel betrayed and we don’t feel we can trust them to protect our future, which is why we’re having to go on strike to make our voices heard, and let them know that unless they change something we will keep striking until they consider our demands.”The crowds were full of school children of all ages who missed lessons in order to show their support for the campaign. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.