The high ground - Melinda M. Snodgrass

Emperor's daughter Mercedes is the first woman ever admitted to the High Ground, the elite training academy of the Solar League's Star Command, and she must graduate if she is to have any hope of taking the throne. Her classmate Tracy has more modest goals--to rise to the rank of captain, and win fame and honor. But a civil war is coming and the political machinations of those who yearn for power threatens the young cadets. In a time of intrigue and alien invasion, they will be tested as they never thought possible.