What the evidence says
When schools notify parents about curriculum materials for sex education, most contacted parents do not follow through to view them. Web analytics from one study found that 27% of contacted parents accessed the digital materials, and only 9% viewed the accompanying animated films (Aventin 2020, high confidence in both direction and magnitude, based on direct website analytics). That reach figure is itself measured against incomplete delivery: only half of the schools in the study confirmed they had sent the links to parents at all, so the 27% figure describes access among parents who were reachable, not among all parents in the sample. A companion survey helps explain the low reach: among parents who did not watch the films, 68% said the reason was that they did not know the materials existed (high confidence direction, medium confidence magnitude — the survey's response rate was only 3%, so this figure may not generalize to parents who did not respond).
On the opt-out side, the evidence is descriptive rather than quantified. One account reports that parents did request withdrawal of their children from relationships-and-sex-education (RSE) lessons when the option was available, illustrating that the right is used, but this comes from a single teacher's anecdotal report and was not systematically counted (high confidence in direction, low confidence in magnitude). Separately, a UK government proposal to remove parental withdrawal rights from newly mandated RSE drew public demonstrations by parents; the source documents that protest occurred but does not quantify its scale or outcome (high confidence in direction, high confidence that demonstrations occurred, but no magnitude is given).
On the administrative side, awareness of opt-out policy is incomplete among the people responsible for enforcing it. In a New Mexico survey, more than 20% of school personnel implementing sexual health education were not familiar with, or did not observe, the state policy that mandates parents be allowed to remove their child from classes with that content (Dickson 2019, high confidence in both direction and magnitude, from direct survey responses).
Across these sources, the evidence base covers notification reach, parental use of opt-out, political reaction to removing opt-out rights, and staff awareness of opt-out mandates. It does not measure downstream outcomes: no source here reports effects of opt-out policy on student health knowledge, behavior, academic outcomes, or on opt-out rates themselves as a percentage of eligible students. That is an absence in the current sources, not a null finding.