Particularly at the beginning of the school year, or after a long break from school, students with anxiety are going to frequently show signs of increased stress. What helps anxious students can frequently benefit all students: provide a warm and inviting classroom environment, establish clear routines at arrival, give students movement breaks, go outside whenever possible, play relaxing music, dim the lights for a quiet time after lunch/recess, practice mindful breathing. Students with anxiety need an organized and predictable routine and warnings when there will be changes.
Teachers can use help identifying common triggers of anxiety like morning meeting when students are expected to give eye contact and greet peers, assessments, large assemblies, the cafeteria, presenting in front of class, and substitutes. The teacher should also know what helps a particular student gain a sense of control and calm (such as a pass to go get a drink). Many students with anxiety benefit from having a list of 3 "safe persons" that they can go to for a break or help, especially when the regular classroom teacher is not available.
Reaching and Teaching Stressed and Anxious Learners in Grades 4-8: Strategies for Relieving Distress and Trauma in Schools and Classrooms (2015) by Barbara E. Oehlberg
This site does a good job of explaining Anxiety in the Classroom childmind.org
Some students with an anxiety disorder will be eligible for a 504 Plan. Some of the common accomodationsor modifications requested are described on these sites worrywisekids.org healthcentral.com healthcentral.com