AB 2127 is good legislation. It requires concussed athletes to use a graduated return-to-play protocol  of at least seven days. It also limits interscholastic football teams to two full-contact practice sessions of no more than 90 minutes each week.
The graduated return to play protocol is recommended by most experts in concussion care. However, concussion care is a highly evolving area. Licensed medical professionals who have knowledge in the treatment of head injuries, but are not experts in concussion care  have on occasion not followed this protocol.
Also, medical professionals can be the victims of poor communication. I have seen return-to-play authorizations filled with conditions. The medical professional means well, but can become the victim of coaches and school personnel who interpret those conditions in a manner that places putting the athlete on the field quickly ahead of ensuring that the child has recovered fully from the concussion. AB 2127 eliminates a number of these return-to-play problems.
Of course, it is not a perfect bill, because, at the same time it solves some problems, it creates a host of new problems. The biggest problem will stem from AB 2127's requirement that the graduated return-to-play protocol be conducted "under the supervision of a licensed health care provider." It is likely that many, if not most, concussed interscholastic athletes will not be directly supervised through each step of a graduated return to play protocol by a licensed health care provider. Rather, the licensed medical professional will likely take reports from school personnel on site or have school personnel monitoring the student as they go through the seven day return-to-play protocol.
This will eventually lead, in all likelihood, to issues regarding best practices and the proper standard of care. Some of these issues will, unfortunately, be resolved through litigation. Other issues will likely be the subject of newspaper exposés on shoddy practices at certain institutions. In short, AB 2127 is good legislation, but even good legislation can be undermined by poor practices of both well-intentioned people, and people who are not so well-intentioned.
AB 2127's football full-contact restrictions is also good legislation. The legislation will need to be fleshed out by regulations defining what constitutes "full contact" and does not. I am sure that the California Interscholastic Federation's State Office will play a key role in crafting the regulations. I further note that many if not most football coaches already restrict the amount of contact during practices, and most coaches place even stronger restrictions on full-contact practices once they begin their season.
Of course, the legislation is needed because athletics needs to have regulations and standards - especially in areas of athletics that have an injury potential. However, while we must acknowledge that legislation is needed, we should not give in to myths about football or any sport. One should not assume that most modern coaches are emulating Bear Bryant when he coached "The Junction Boys."
Finally, I note that there is one downside to AB 2127: it only regulates interscholastic sports in public schools, private schools, and charter schools, both middle schools and high schools. However, it does not address camps and clubs. AB 2127 could have the unintended consequence of driving more football players to private camps and clubs. The best of these camps and clubs engage in best practices. For example, organizations that follow Heads Up Football and many Pop Warner groups adhere to tight standards.
However, if AB 2127 leads to an expansion of camps and clubs we could start seeing more organizations that either ignore or are unaware of best practices. This could lead to a situation where schools are subject to ever tightening regulations while students escape those regulations by finding organizations that do not.