All coursework is marked by your teacher(s). Within your school someone (such as the Head of Science) is responsible for ensuring that all the science teachers in the school mark work to the same standard.
Around Easter time in Year 11 the final mark total for each student is calculated. The school then compiles a list of all the pupils and their scores from the highest to lowest marks. This list is sent to the External Moderator. From this list the Moderator will choose a random selection of work to check from across the range of marks. Within a few days of receiving the request for samples from the Moderator the school must send off the relevant pieces of coursework.(This is why it is important not to lose your coursework! Often teachers won't let you have it back once they have marked it.)
The Moderator checks that each piece of coursework has been marked to the correct standard. If the marks are wrong by a significant amount then the Moderator will adjust the marks for the whole school - this could be either up or down. Hence you can not always be sure that the marks you have been given by your teacher are the final marks used to calculate your GCSE grade.