You would absolutely be letting down your other students. You should have a set of lesson plans and instruction based on each grade level and their individual standards. 8th graders should not be being taught 5th grade standards unless they are that far below grade level; even then you should have a plan to increase them closer to their own grade level. Create a plan for specific conferences with each grade level on different days and specific times. For example: Set up a center type rotation that occurs regularly every day and week. Monday you teach 5th grade math for the first twenty minutes; give them independent practice work and a way to check their answers using technology. Then work with the next grade level and subject. If certain students are on the same academic level in a particular subject you can combine the groups. Design small groups for instruction with a rotating teacher group and independent practice; additionally plan days for remediation of difficult skills or where you know the student's weaknesses lie. Fridays can be used for assessing growth, accomplishments, and celebration. They should be exposed to grade level expectations however; an easier vocabulary level or lowering the amount of work expected will aid students in meeting grade level expectations as well.

I have not had your same experience, but in a general education classroom I have often had students with a variety of skill or abilities. I find a great way to differentiate instruction and meet the needs of all of my learners is to use learning clubs or small groups. Each learning club area would feature different tasks for each level of learner according to their needs. You may be able to change them out as needed for different concepts based upon ability. You are able to work with smaller numbers of students while having other groups work on specific tasks to their own ability levels. All of your students will still receive small group instruction and even one-on-one attention if needed within their own group.

I have a room very similar to this, but what I've found is that many of those 5th grade standards are recursive--meaning they're built upon to form the concepts for the higher grades. Knowing this, I would introduce the concept based on the fifth grade standard and then group the students based on ability and remediate by building on that fifth grade foundation in the groups. It's very difficult to do, but you will be surprise how shaky some of your students foundations are!