The Academy of Our Lady of Peace high school in North Park closed temporarily on Thursday, nine days after it reopened, because two students tested positive for COVID-19.

The closure is one of several that some school and health experts feared could happen in San Diego County, which gave the green light for all schools to reopen on Sept. 1 because the county’s COVID-19 case rate was improving.

But the rate is worsening again, so much so that the county is close to meeting the threshold that would put it at risk of moving to the lowest tier of the state’s tier-based reopening system.

The two students tested positive despite the Catholic school’s efforts to enforce safety measures, which included plexiglass shields around desks, hand sanitizer before every class, daily deep cleaning of rooms and an on-campus health center staffed with contracted medical personnel.

“We recognize that despite our best efforts the girls are struggling with maintaining the rules of physical distancing both on and off campus,” school leaders said in a letter to families Wednesday.

The two student cases are not related, according to the school, which has 750 students.

In an emailed statement, the school said the health and safety of its students and staff are its top priority.

“We are adhering to the strictest CDC, local and state protocols and taking extra steps to ensure the safety of our community, including transitioning to a hybrid learning model to reduce the density of students on campus,” the school said in the email.

The school switched from in-person learning to distance learning on Thursday. It will switch to hybrid learning on Sept. 17, where students will attend class in-person for two days a week, and it will continue hybrid learning until mid-October.

“This will allow time for the community to separate, practice physical distancing, and reflect on the importance and privilege of our time together on the OLP campus,” school leaders said in the letter to families.

Some health experts have said it is inevitable that COVID-19 cases will surface at schools, considering the number of schools in other states that allowed schools to reopen that already have had to close again due to COVID-19.

Experts have stressed that community spread is linked to spread in schools, and countries that have successfully reopened schools had significantly lower coronavirus case rates than what California considers as safe enough to reopen schools.

San Diego Unified and school districts in South County, where COVID-19 is disproportionately concentrated, have been adamant about not reopening until community spread of the coronavirus decreases further, to avoid having to close again or put students and staff at risk.

The county logged 6.9 new daily cases per 100,000 residents as of Wednesday. If the county logs a rate of more than 7.0 for two straight weeks, San Diego County will fall to the lowest tier of the state’s reopening system, which is the purple tier.

In the purple tier, middle and high schools are not allowed to reopen, but elementary schools can reopen with a county-approved waiver.

However, no matter how much the county’s case rate increases, all schools that reopen before the county falls to the purple tier will be allowed to remain open, according to state guidance. The guidance says those schools should begin testing staff or should test their staff more often for COVID-19.

According to state guidance, if 5 percent of students or staff in a classroom test positive for COVID-19, an entire classroom should close.

A school should close if there are multiple cases in multiple classrooms at a school, or if 5 percent of its students or teachers test positive within a two-week period.

A school district would need to close if a quarter of its schools close within a two-week period.

State guidance requires schools to immediately notify county health officials, staff and families of any exposure to a positive COVID-19 case at school.