Arranging for guitar is a very complex and involved subject, so we will handle it in small chunks. Here are some basic 'rules'. 

These are generalisation but are usually applicable to most high school aged guitarists. There are, of course, exceptions.

  1. The guitar sounds an octave LOWER than it is written.
  2. The open strings are tuned high to low E B G D A E. (Other tunings are possible but standard is still 'standard'!)
  3. Cluster chords of 2nds may be unplayable. 
  4. Keys of F Bb Eb Ab Db Gb are very difficult for most young guitarists. (Usually 5th - 6th grade standard)
  5. Sharp keys are generally preferable.
  6. DO NOT just remove the bass clef from a piano part and give it to the guitar player.
  7. Wherever possible add chord symbols to the part.
  8. 3 octave scale passages at fast tempos will always cause a problem.
  9. Very fast arpeggio lines and octaves or double stops may also be tricky.
  10. It is best not to write out an arpeggio picking pattern unless you transcribe the guitar part exactly. 
  11. Always remember that just because something sounds good on piano or a guitar MIDI track doesn't mean it will be playable on guitar.
  12. A page with the lyrics and chords above them is often not enough information for a beginning guitarist. They need to know how to strum it, how many bars of each chord, any shots or stops etc. 
  13. Take the time to scratch out a piece of music for the guitar players. Even it is sparse, you will be doing your guitar players a great service by helping them to learn how to follow a chord chart.
Now that we have covered some of those basics, let's look at some specific examples.

Since the guitar sounds an octave lower than written, it can be quite 'muddy' in the mid range (notes on the staff) so you might want to write it an octave higher to let the notes ring. If guitar is playing unison with a flute, trumpet, clarinet or violin it might be nice to write it in the middle range to fatten the sound. If writing an octave higher, use 8va symbol rather than lots of ledger lines.
The range on a standard guitar is about 3 octaves and a 7th. Acoustic players will generally have problems
reaching the higher frets unless the guitar has a cutaway. 

Here is an example of a phrase that would be difficult to play. 

The 3 note clusters played as eighth notes make this a difficult phrase. Stretching the hand over 4 frets is also an awkward move.

Simply drop the lowest note off and the phrase becomes playable as diatonic thirds.
Another alternative is to have another guitar play the lower notes in the phrase.