When an older sibling starts school it can be a difficult transition for a younger sibling or siblings left behind at home. It's an adjustment to the little routines that have been built up if you have had all the children at home together, and even if they are used to seeing an older child go regularly to nursery or pre-school it's still a big change for everyone.
When my oldest started school my daughter became very clingy, and she clearly missed having her brother at home to play with and keep her company (even though it often seemed as though all they do is fight over their toys!)
Here are some thoughts about things that we've done to make things easier:
Involve the younger sibling in the initial preparations for school. Buy uniform together and let them watch while the older child tries it on while you talk about school.
Take younger children along to any events at the school like plays or class assemblies, so that they can have a glimpse of the older child in their new setting.
Having to drop off and collect the older child from school each day will have a huge impact on the daily routine. It may mean that you can't do some activities that you used to enjoy, for example visiting a friend for the afternoon. You will be able to find other things that you can enjoy, for example we often stop off at the local library or playground on the way home from school.
The school run may have an affect on nap times. My daughter dropped her regular nap long before her second birthday, but having her in the pushchair on the school run often meant that she dozed off, becoming a problem at bedtime when she wasn't tired. I tried to encourage her to walk some of the way when she was looking tired, and gave her a pot of snacks and a drink while she was sitting in the buggy. Usually she perked up on the way home when my son was with us and she scooted some of the way home with him.
Find a new activity to enjoy with the younger child, perhaps something like swimming that it is easier to do when you just have the one child with you.
Plan activities with the younger child at home for when the older child returns from school. Think about setting up some craft activities or a sensory tub that all the children can enjoy together. Try to make the couple of hours after school a fun time where you all do something as a family.
The older child is likely to be quite tired when they return from school and they might not be in the mood to play with younger children. They might also have homework or reading that needs to be done. This can be tricky to manage, as you need to keep the younger child entertained while giving the older child some quiet time. This will get easier as stamina improves and the new routine settles down. I often let my son sit on the sofa with an iPad while I play with the younger child in the same room.
There might be elements from school that you can bring in to the home. My daughter loved having her own bag that we called her 'book bag' and put reading books into. She likes playing 'going to school' with her teddies, pushchair and bag. I've also sometimes made her a lunchbox with her sandwiches in. It also all helps with her own transition to school.