As per my knowledge Varna system is established so that person can chose his profession based on his characteristic. Brahmin always engaged in knowledge and worship whether it is vedic, medicine or warfare knowledge. They did not involve in killing of human being. Parshuram and Drona are exception. And Kshatriya are class of King, Administrator (minister), soldier etc.
Now Suta is inter-mixture of Brahmin mother and Kshatriya father. So because of DNA characteristics he was not allowed to participate in a war as a soldier/warrior as it involve killing. But they can choose as a charioteer to fulfill his Kshatriya desire of seeing battle.
I think at the time of MB children are forced to follow Varna system in choosing profession but not that much. Those who want to be soldier can train under Kripacharya gurukul. Also many other princes come to Drona ashram for higher education in warfare. He chose student based on merit. Karna was also student of Drona. Drona taught Brahmastra to deserving student like Arjun, Yudhistir and Bhima but when Karna asked for Brahmastra he rejected based on his caste. But his real intention is to make Arjun superior than other and he know Karn envy of Arjun. Later he taught Arjun Brahmshira after Drupad defeated by Pandava.
Everybody knows Karna is adopted by Adhirath. Even Duryodhan said in Kurushetra that he must be son of pure Kshatriya parents. Bhism did not allow in Karna because he did not want brothers kill each other. Also Bhism and Karna together will be too much for Pandava.
Kripacharya did not allow Karna in Rangbhoomi because he calculated Karna is matched to Arjun in archery and if Arjun lost it will be insult to Royal family.
Draupadi(kmg) rejected Karna because she did not want to go with lower caste. I think Karna was invited OR Swayamver is open for all but preference is given to King and Princes. After all King failed Karna stood up and he was not stopped by anybody untill Karna lift the Bow and stopped by Draupadi/ Dristdyun (whoever).
Edited by Neutral2 - 10 years ago