Karl XII recieved a very thorough education, incuding military matters. A set of very skilled and experienced tutors were hired for the young prince's education, and his father, King Karl XI, took an active interest and at times oversaw the education himself to ensure the quality.
He was originally tutored in military matters by Carl Magnus Stuart, who had briefly served in England, worked extensively on fortification design and observed the Austrian army in its war against the Ottomans 1685. He also toured Italy, Switzerland and the Netherlands to study the lastest military developments. Later the young Prince and then King would also be tutored by Carl Gustaf Rehnskiöld, one of the most brilliant Swedish generals and perhaps one of the best of his time.
Rehnskiöld had served in the Scanian war 1676-79 together with the King, Karl XI and had been part of the 6 000 men force Sweden rented to the Dutch 1691 during the Nine Years' War (or Pfalzian Succession War) and experienced first-hand the effectiveness of the offensive French infantry tactics, which relied on fire, advance and then charge.
Rehnskiöld and Stuart, together with King Karl XI would be the fathers of the Swedish "gå på" tactical school that they developed, based on earlier Swedish tactics and the offensive French tactics as experienced by Rehnskiöld. The young prince often accompanied his father, Stuart and Rehnskiöld as they tried out various tactical solutions, using the Life Guards regiment in Stockholm in mock battles using blank ammunition, building various fortifications and terrain features at the royal grazing meadow Ladugårdsgärdet (today a public park in Stockholm). He also accompanied his father on several inspection trips, where they would travel simply and at very high paces with very few followers to catch officials and officers off guard and reveal corruption and neglection of duty. He also partook in generalmönstring, a system in which the indelta (alotted) Swedish semi-professional army held larger musters - entering new soldiers into the lists, retiring older ones, inspecting equipment, supplying new or replacement equipment but also held larger exercises - both in drill and mock battles including several regiments.
By the time he was 18, the young King had what was probably one of the best military educations in Europe at the time. And while he repeatedly showed an aggressive, offensive spirit, driving his men onwards, he also assigned planning to more experienced officers. Stuart negotiated with the Anglo-Dutch fleet commanders and planned and commanded the landing at Holbaek which knocked Denmark out of the war 1700. The King approved the plan and partook in the landing himself, but the main work was Stuart's.
Likewise, while the King rejected pleas to await reinforcements and supply before the Battle of Narva 1700 and drove his army of about 11 000 hard towards the Russian army, the plan of attack was actually developed by Rehnskiöld. It seems like the King's education was continued in practice in the field - Rehnskiöld would also plan the Crossing of the Düna in 1701 and the Battle of Kliszow 1702. It was not until December 1702 that Rehnskiöld would leave his de facto role as chief of staff of the young King to lead an independent command. During the Grodno campaign 1706, the young King led the main army to out-manouvre the Russian army in eastern Poland-Lithuania, separating it from its Saxonian allies, then the cavalry form the infantry and then forcing the Russian infantry back into their winter camp at Grodno, sans supplies and food, laying siege to them causing about 17 000 Russians to perish from disease and starvation - while 5 000 Russians managed to flee through a swamp and join with a large part of the Russian cavalry to form a nucleus for the Russians to rebuild their army around, it was nevertheless a great victory in manouvre warfare.
At the same time Rehnskiöld wold annihilate a Saxonian army at Fraustadt.
So, to answer your question.
While Karl XII had a very thorough military education by some of the finest generals of the era that had a lot of experience, including his father the King and the final say, he did let his more experienced generals plan his operations early during the Great Nordic War while he retained control of the overall strategy and wether to deliver battle or not.
He inherited a tactical system developed by his father and the same generals that tutored him, from a combination of French and Swedish experience and only modified it slightly, updating the infantry's tactical manuals of 1696 and 1701 in 1708, changing the distance of salvos fired from 70 and 30 paces to 20 and 10.
He did have experience with mock battles and exercises, both from the Life Guard experiments and the generalmönstrings as a child.
After his long stay in the Ottoman Empire, the army had started to phase out the pike, which he re-introduced as vital to the offensive Swedish tactics - the Swedish army at Helsingborg 1710 is said to have "tumbled over" the Danish infantry due to charging in formation with pikes, with the Danes unable to fight back with their shorter muskets with bayonets attached.
Sources:
Karolinen Rehnskiöld (The Carolean Rehnskiöld) by Jan von Konow.
Karolinsk och Europeisk stridstaktik 1700-12 (Carolean and European combat tactics 1700-12) by Gunnar Arteus.
Armfeldts karoliner (Armfelt's Caroleans) by Anders Hansson.
Kampen mot övermakten (the struggle against the odds) by Boris Grigorjev and Aleksandr Bespalov.