Every site is different? Some sites have the labs set up in a very small room with not enough room for chairs. Other sites have adequate space and a chair at the IV push station. To answer your question, be ready for any different variation. Even though someone tells you that there is a chair at every different site... practice both ways.
Before I took my CPNE.. i practiced IV push while standing. And I'm pretty tall 5'10" and it felt like I was going to break my back sometimes, especially if it was IV medication that took really long time (3mL over 3 minutes). Those 3 minutes is a very long time to stand and concentrate. During my test, there was a chair but to me it didn't even matter cause now it was way easier for me to do the IV push station under any and all situations!
If you have back issues... don't hesitate to ask if you need one. But I'm also concerned if it is a small room with all the stations and 6 students testing, can anyone guarantee having a chair?