A group of California junior high students were caught forming a swastika with their bodies on school grounds and exchanging racist and violent messages on a group chat, administrators said.

The scandal at Matilija junior high school, which culminated in an emotional meeting with parents and school officials Monday night, has sparked intense debate in a region that has experienced a sharp increase in reported antisemitic incidents.

The middle school in Ojai, a small city 80 miles north-west of Los Angeles, told parents in a letter in December that officials had discovered photos showing “nine students laying on the field together to form the shape of a swastika during lunch”. Administrators said the images appeared in a group chat that was active in November and December and featured “racist, sexually inappropriate and threatening commentary”, including one student’s call “to bring knives to school”. The photos and texts have not been released.

Cyndi Silverman, regional director of the Anti-Defamation League (ADL), who has been working with Ojai school officials, told the Guardian that there were 28 students on the group chat, which featured a wide range of hateful content: “There were a number of texts that were anti-LGBTQ, antisemitic, anti-black, anti-Latino.”

Antisemitic incidents in the US have recently surged to the highest level in two decades, according to the ADL, which documented 457 cases in K-12 schools in 2017, a 94% increase from the previous year.

The local ADL for Ventura, Santa Barbara and San Luis Obispo counties responded to five antisemitic incidents in 2016, followed by 19 in 2017 and 21 in the last year, Silverman said. Ventura County is a “hotbed for white supremacists and the ‘alt-right’ movement”, she added.

The Ojai controversy follows numerous reports across the country of increased school bullying echoing the hateful rhetoric of Donald Trump, and a rise in harassment, abuse and offensive language by students on Instagram, Facebook and other digital platforms.

Of the 28 students in the chat, 12 received a “disciplinary consequence” due to their participation in the texts or involvement in the swastika incident, the superintendent, Andy Cantwell, said in an email Tuesday.

The Matilija student body is 60.6% white, 31.9% Hispanic or Latino, 1.9% Asian and .7% black. Police investigated and said there were no criminal violations, and the school told parents there was no “active threat” to students.

The incident, however, inspired others to come forward with their own stories, said Silverman, who moderated this week’s public forum with the school and families. One mother who spoke at the meeting said that her son, whose father is Jewish, had received a threatening antisemitic text message that referenced the Holocaust, according to Silverman. Another speaker, a young man of color, talked about experiencing racism and being “jumped”, she said.

An assistant superintendent also said at the meeting that the district had seen an increase in students with anxiety and depression, according to a reporter with the Ventura County Star.

Some at the meeting voiced concerns about the culpability of the parents of the students in the chat. Silverman said she felt the problem was wider than individual families: “Biased behavior is learned from your environment. We have to look at the whole community at this point and the broader country … before we blame parents. We’re all responsible.”

The local ADL has responded to recent cases involving graffiti with Nazi imagery and a swastika at a synagogue, as well as students in a different district caught making violent misogynistic comments in an online chat, she said. One particularly shocking case involved a five-year-old making violent antisemitic statements to another child, she added.

In 2014, a college student in Santa Barbara went on a murderous rampage after expressing violent and misogynistic views.

Brianna Moffitt, director of education and community outreach with the local ADL, said the Ojai case rippled across the region: “It’s heightening everyone’s awareness that we have hate in our community. Some of these things are being taught and being learned in our community.”

Moffitt said she hoped the case would encourage other school districts to be more proactive in addressing bias and hate speech.

Cantwell, the superintendent, said: “As a district we are developing strategies to promote anti-bias and culturally responsive learning environments,” adding that antisemitism was a “national problem”. “We ask the Ojai community to continue to join us in sending the message to all young people that this is wrong,” he said.