I forget who first said this but I have used this quote often: “School should not be a place where young people go to watch old people work.”
There is nothing that requires you to correct and grade everything a student does. In fact, marking everything is not good – because a learner’s early mistakes and errors are now incorporated into their grade. Giving formative feedback is necessary, but there are some strategies to help you reduce the workload while not diminishing the learning that is taking place.
The easiest manner to accomplish this is to ask students for some written work on exactly what you want them to take from your lessons. Whether it be observations, conclusions or a self-reflective piece, you will get what you need without having to read entire notebooks.
Mark notebooks only periodically. Give students checklists for students to complete and have them attach this to their notebooks. They can even self-assess their work using a rubric that you can supply. My teaching was revolutionized by an online rubric maker that is still up and running: http://rubistar.4teachers.org/ You can quickly and easily make rubrics and save them.
Peer assessment is an excellent learning strategy that also reduces the work for you. To read over someone else’s work and give them feedback tests your own understanding (as you have probably observed in yourself!). I would incorporate this into almost everything your students are doing. This can be done in pairs or in groups – just spend some time teaching students how to do these assessments. I used to give small, coloured slips with questions that would guide the students in their assessment and have it attached it to the work. These coloured papers are easy to thumb to in a notebook for quick checks.
Hope this helps!