Theme: SAT prep - There isn't a unifier so my take is that the last word could describe getting a student ready to take something like the SAT's. You're welcome to post a different slant; please do.
17A. Older name for a passenger bus : MOTOR COACH
24A. Wedding gown follower : BRIDAL TRAIN
34A. Like one resisting innovation : OLD SCHOOL
50A. Pirate Blackbeard's real name : EDWARD TEACH
58A. Hole-making tool : POWER DRILL
Argyle here. Looking over past puzzles from David I see he often doesn't give a unifier. The grid is notable for the strong corners,(we like corners). COACH, TRAIN, SCHOOL, TEACH and DRILL answers are all consistently used as nouns in the theme entries at a different context.
Across:
1. Peru's __ Picchu : MACHU
6. Angle iron : L-BAR
10. Highest point : ACME
14. Kindle download : eBOOK
15. SeaWorld performer : ORCA
16. Ellington's "Take __ Train" : THE 'A'
19. Glass darkener : TINT
20. Responded in court : PLED
21. Cape NNW of Cod : ANN
22. Saguaros, e.g. : CACTI
23. Covered up : HID
27. Place in quarantine : ISOLATE
29. Legal thing : RES
30. Came down with : GOT
31. Kate, before Petruchio's "taming" : SHREW. Shakespeare, "Taming of the Shrew".
32. Bit of legislation : ACT
33. U2 lead singer : BONO
38. Die dots : PIPS
41. Thumbs-up : YES
42. Best man's offering : TOAST
46. Santa __ winds : ANA
47. Fellows : MEN
48. Stir-fry vegetable : SNOW PEA
53. Rank below cpl. : PVT.
54. Believer in the Great Pumpkin : LINUS. from "Peanuts".
55. NYC airport : LGA. LaGuardia Airport (code LGA) in New York City.
56. Narrow opening : SLIT
57. Installed, as carpet : LAID
61. Years, to Nero : ANNI
62. Wows, and how : AWEs
63. Stone marker : STELE
64. Droops over time : SAGS
65. Peel in a cocktail : ZEST
66. Filled with cargo : LADED
Down:
1. Tennessee home of the NBA's Grizzlies : MEMPHIS
2. Do away with : ABOLISH
3. French department that translates to "golden slope" : CÔTE-D'OR
4. Robin __ : HOOD
5. Kiev is its cap. : UKR. (Ukraine)
6. Canadian coin nicknamed for the bird on it : LOONIE
7. Lego or Eggo, for example : BRAND
8. Duke Univ. conference : ACC. (Atlantic Coast Conference)
9. Stadium shout : RAH
10. Rose essence : ATTAR
11. Lake Michigan metropolis : CHICAGO
12. Bring up : MENTION
13. Chip away at : EAT INTO
18. Golfer's ride : CART
22. Dollar divs. : CTs Cents
24. Cry out loud : BAWL
25. Curved foot part : ARCH
26. "Dallas Buyers Club" actor Jared : LETO. IMDb
28. Some summer babies, astrologically : LEOs
32. Summer coolers, for short : ACs. (air condition)
33. What winds do : BLOW
35. Like Easter eggs : DYED
36. Emailed : SENT
37. Texter's "From a different angle ..." : OTOH. (On The Other Hand)
38. Spanish rice dishes : PAELLAs
39. Gary's home : INDIANA
40. Hocking : PAWNING
43. Answered a help-wanted ad, say : APPLIED
44. Whence Rossini's barber : SEVILLE. "The Barber of Seville"
45. Spilled the beans : TATTLED
47. Dalloway's title : MRS. Mrs Dalloway is a novel by Virginia Woolf published in 1925.
48. Most judicious : SAGEST
49. Virg. neighbor : N. CAR
51. German cars : AUDIs
52. Actor Cary : ELWES. The Princess Bride, Robin Hood: Men in Tights, Days of Thunder, Bram Stoker's Dracula, ...
56. Latina lass: Abbr. : SRTA. (senorita)
58. Peace, in Acapulco : PAZ
59. Be indebted to : OWE
60. High-speed www option : DSL. (digital subscriber line) not high speed.
Argyle