Reading Block Woes

Ugghhh, this has been a very frustrating first 5 weeks of school. Don't get me wrong, I love my class, they are great! It has been frustrating finding a rotation that will work for my reading block. I have 3 reading groups that I meet with 4 days a week for 20 minutes at a time. My lowest group consists of all my students who are pulled first thing for school wide small group reading intervention. That way they get a double dose of reading, once with their intervention group and then again with me. Now, herein lies the problem, my students that are in that group are not all at the same level, there are 4 of them that are low readers, average of about a 2.5 reading level. The other 3 students are really low readers, with an average of about a 1.5 reading level. So I feel like if I go too slowly or provide activities that are on a lower level then I am not challenging the "higher" students in that group, but if I move faster and provide more challenging activities then I am not meeting the needs of the 3 lowest. So I am debating on whether I want to split that group up and create a 4th group. But if I do a 4th group then I have to cut 5 minutes out of each of the other groups to create an equal amount of time for each group. So I am wondering if I will be doing anyone any good if they only have 15 minutes a day of small group time. I have thought about making the 4th group and then alternating days to meet with the two higher groups so I am still meeting with each group for 20 minutes but I know how I am and I would not be happy with that arrangement, I am the type of teacher that MUST meet with my kids every day that I do small groups. Also, considering that my 2nd highest group consists of students who are still reading below grade level, I would not be comfortable meeting with them 2 days a week instead of 4. The good news is I don't have to figure it out right now, because right now I have a senior intern so I can divide up that group and have the 4 "higher" low students work with her while I work with the lowest 3, but I only have my student teacher until December, then I have to figure it out on my own.