A.,
We have the same issue with my daughter in first grade. Thankfully she has a teacher that is willing to give her more challenging assignments - different spelling list, lets her read chapter books, attend special groups, etc.
I also do things at home on the weekends and breaks to give her a challenge. I would caution you not to move your child too far ahead in the curriculum, but go in more depth in the curriculum. Moving ahead just causes more boredem at school, going in depth challenges their mind. Some subjects are easier than others. Let me give you some examples...
Social studies & history - find out what they are learning about and get books / videos from the library & off the web about the people, places and events. Find out the interesting details that the text books never seem to contain and do some family lessons at home.
Math - Here is a subject where it is hard not to jump ahead. So when my daughter started learning double digit addition at school, I showed her that there are several ways to add things on paper, how to do it with manipulatives and also how to do it in her head. You can also teach base 10 theory and even other bases. (They most likely won't do it in school until college, but I have seen it taught to children and they actually get it faster than the adults do because their mind isn't boxed into "base 10.")
Reading - I let my daughter have at it in this area, whatever suits her fancy. Right now it is Harry Potter.
Science - You can always do experiments at home that compliment what is happening at school. If you have science museums or children's museums in your area, go visit. Many even offer tours or classes.
Use the resources in your area. My daughter likes attending a young scientist class one evening a month. We take LOTS of field trips to different educational places and try to add an educational aspect to our vacations. Example: We are going to rural Mexico for my bro-in-law's wedding this summer, so now we are studying Incas and Aztecs at home and we are going to tour some ruins when we are in Mexico.
Some teachers allow their gifted students to do a "great brain" project where they research a topic of their own interest at home or in spare time at school, then when they have it put together they present it to the class. This teaches a lot of life skills and challenges the student without pushing them ahead in curriculum or creating unrealistic amounts of extra work for the teacher.
I considered homeschooling at one point, but realized my daughter is much better off with her peers in a classroom. My husband helped me realize that it wasn't in her best interest to isolate her or to only group her with those on her intellectual level. She will spend her whole adult life working with people are either intellectually superior to her or who may struggle in areas she doesn't. She needs to learn how to deal with both ends. I recognize that my daughter's teacher can't do it all, so I try to work with her, not compete against her.
Also, she has weaknesses like everyone else. Her handwriting is terrible and she is really absent minded about repetative or mundane tasks. So we can focus on improving those things as well. I hope you are able to find a good fit your sons, whatever it may be.
S.