The US Marshals were there on site and stopped people from intervening. Odds on them being the reason the police chief ordered his men to stand out? Also they could have engaged him before he got to the school but didn't

https://www.ksat.com/news/local/2022/06/05/uvalde-mother-who-ran-into-school-describes-being-handcuffed-saving-kids-during-shooting-im-going-in-there/

The US Marshals were there on site and stopped people from intervening. Odds on them being the reason the police chief ordered his men to stand out? Also they could have engaged him before he got to the school but didn't