I think that OP is misconstruing the reasons behind this notification requirement. This notification has to do more with a "cover your ass" policy than lowering expectations for seniors.
The California Education code [1] requires school districts to develop procedures to notify parents of a failing grade. All the CA districts that I worked for had some procedures in place for when and how notify families of failing grades in response to this law. From a quick search, it seems that SFUSD policies 4.2.5 [2] and 4.2.6 apply in this case. The policy clearly states that teachers have to notify parents of grades either 1 or 2 times during the semester (i.e., mid-semester report cards-if you went to school in CA you will remember receiving those).
It seems that OP's school was on a 9-week reporting period, requiring teachers to notify parents of grades mid-semester and that their school added a requirement of filling out an additional form outlining that the student was in danger of failing the class specifically for seniors (probably because most families stopped looking at report cards for seniors). This form is required to be in compliance with the CA ed code as parents have the right to appeal failing grades and not being notified is probably an enough reason for the district to change a failing grade to a passing one in case of a lawsuit/complaint.
I would imagine that failing a class is more "high stakes" for seniors than other students, so they have more paperwork involved for seniors if you want to fail them. Families might be more "litigious" if a student ends up failing a class required for graduation. It is simpler to complain to the board/suing that having to repeat a school year.
The bottom line is that the CA ed code gives teachers final say in grades and even the superintendent cannot change a grade without the teacher's consent. On the other hand, the CA ed code gives some rights to parents to appeal grades and has some notifications guidelines so they shouldn't be surprised of failing grades. OP will be able to fail as many seniors as they want, but they just have to notify their families that they are in danger of failing the class at some point before the end of the semester. At the end of the day, if they have enough evidence for failing a student (that would stand up during a public board meeting/lawsuit), notifying the student's family 9 weeks before the end of the semester shouldn't be too much of a burden.
Links:
[1]: https://leginfo.legislature.ca.gov/faces/codes_displaySectio.... [2]: ]https://www.sfusd.edu/services/know-your-rights/student-fami...