The Time Princess: The Time Masters’ Chronicles, #1

You might think it's another ordinary day at school but just around the corner there may be a force that can change your life forever. Klaudia Cay is a tetrathlon champion. She should be happy but has problems with her mother, brother, and a new girl in her class. 

That pales when she discovers she is a Time Princess and is 'Called' to the year 1643. Her mission is to prevent the assassinations of England's King Charles 1 and Oliver Cromwell. Klaudia finds herself fighting for her life on the bloody battlefields of the English Civil War.