I don't think we have an official rule but some teachers do specify no siblings. Basically events in the gym, cafeteria, or auditorium are open to all and things in the classroom are up to teachers. When my oldest started kindergarten I had dd1 with me when we toured the library. The librarian asked me to volunteer but I said I couldn't due to youngest dd. A week later I got a call saying that the principal approved my volunteering with dd with me. I did help for the school year while dd slept in a stroller.
Both dd's had the same kindergarten teacher. Siblings were invited to all classroom events as long as we took them out if needed. I was surpised when my youngest was in the class and the kindergarten teacher asked my oldest dd's teacher to send dd down for a special classroom event. My kids are in the spring concert this year (alternates with winter) but for the winter concert this year there was a notice that said siblings in the school could not attend events like concerts during school time. Things like Title 1 talks and family nights always include younger siblings.
Both dd's had the same kindergarten teacher. Siblings were invited to all classroom events as long as we took them out if needed. I was surpised when my youngest was in the class and the kindergarten teacher asked my oldest dd's teacher to send dd down for a special classroom event. My kids are in the spring concert this year (alternates with winter) but for the winter concert this year there was a notice that said siblings in the school could not attend events like concerts during school time. Things like Title 1 talks and family nights always include younger siblings.

How are you supposed to concentrate on helping a child read, running a center, correct papers, etc when you have a younger child who needs to be watched? 
There are some occasions when this is true.