Students from a Texas school that was hit by one of the worst mass shootings in US history returned to class Tuesday under tight security, American media reported.

The May massacre at Robb Elementary in Uvalde -- in which an 18-year-old shot and killed 19 students and two teachers as police waited more than an hour to confront him -- sent shockwaves throughout the country.

Classes did not resume after the attack, and the start of the new school year was delayed by a month to allow still-traumatized students more time to prepare.

The children will not be returning to the original Robb Elementary building and instead began their new classes at another school -- still with the same name -- about half a mile (one kilometer) away.

In an effort to reassure shaken parents and families, administrators have implemented several security measures at the new building, including a perimeter fence, surveillance cameras and guards.

The school has also said it is making mental health counselors available to students, and local media reported that 10 comfort dogs will be present at schools throughout the Uvalde district for the first few weeks of the year.

Despite these efforts, some families remain wary, with several telling CNN they hesitated to send their kids back because the emotional pain of the last several months is still too strong.