I applaud you for being proactive in reaching out to parents.

There are many resources recommended by parents on the Recommended Resources forum... so many that it may take a bit of sifting to find the right resource for a particular student. Beyond academics such as 6th grade Math, ELA, and social studies, there is the readiness and ability of each student to consider. Children may need or benefit from a teacher who is an advocate, and a partner with the parents.

For example, some students may have a keen interest in a particular area and be working on (or have mastered) material which may be considered 1, 3, even 5 grade levels ahead of what may be prescribed by common core standards.

As another example, some students may have diagnosed (or undiagnosed) learning disabilities and/or neurological differences such as ADD/ADHD, Autism/Asperger's, dyslexia, dysgraphia, dyscalculia, dyspraxia. From the outside, these children may appear willfully lazy or disobedient but on the inside, they may be working very hard to keep up and stay on track. They may need or benefit from IEPs/504s, and specific supports for remediation, accommodation, and instructional differences. This article from Reis & McCoach (UConn), titled Underachievement in Gifted and Talented Students with Special Needs, includes a table of "Characteristics Which Hamper Identification as Gifted."

How are classes managed at your school, for example, are there state laws about gifted education? What are the school or district policies and practices for gifted services? Is there a coordinator of gifted services? Is full grade acceleration and/or subject acceleration allowed? Are there pull-out services? Is there flexible cluster grouping by readiness and ability... across grade levels... or within the classroom? These factors plus other constraints may help determine which resources you (and/or your school administration) choose to make available to students.

If you've not already joined the Davidson Educator's Guild, that may also be helpful.

ETA: The book How the Gifted Brain Learns presents research-based information which may be helpful to teachers: gifted individuals exist, differences are brain-based, gifted pupils tend not to just be kids who happen to be ahead a little bit.

Also adding a link to the thread Best practices in teaching.