The Academy - S3-E6
Plot hole: When Ahsoka rushes to save the students when they arrive to meet the prime minister, she attacks several secret police officers, who flee afterwards. However, we later see her standing next to the prime minister acting innocent when she pretends to side with him. The officers she fought earlier would no doubt have told Almec what side Ahsoka is really on, so her ploy never should have worked to begin with. (00:11:55 - 00:13:40)
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.