Different schools take different holidays. I have looked up my former school - they are off Monday 11th to Monday 25th April whilst the school in the village where I currently live have Monday 4th to Tuesday 19th April off. London will be busy throughout this period.
All shops over 3,000 sq ft have to close on Easter Sunday by law. Cafes and restaurants can open. Most tourist sites will be open throughout the long weekend, but check online before you go.
Often, engineering work is undertaken on the rail tracks over the long bank holiday weekend - check tfl.gov.uk before you travel.