The events you describe are simply too dangerous to continue with. IF the senior instructor was present and supervised the class, then run from that dojo quickly. If it was a senior student supervising, it would be worthwhile to speak to the Sensei.
Your safety is the responsibility of your Sensei, your seniors, your partners and yourself. We did a similar class last night to help prepare one of our yudansha for his nidan test. A brand new student started and participated in the class. When it came time for him to take falls, nage would stop and assist him in taking a safe forward roll from a kneeling position. There is no excuse for a situation as you described it! You would be safer learning to disarm bombs or something.