Granting residency at a Bachelor's degree level might encourage people to major in something "easy," but perhaps not truly elevate productivity - at this level, only a job license would be issued.
They would then be given 3 years before their license expires if they don't continue on to pursue a Master's degree. Once it is obtained, permanent residency would be granted.
This two-step, multifaceted approach would encourage undocumented immigrants to pursue education and careers beneficial to American society and economy, rather than living their lives hiding from the government and being unproductive.