Vomaxx is right on the money.
Oftentimes nobles who could afford to get out of fighting would just pay wehrgeld, or money for service- for instance, they would either hire out mercernaries or soldiers either trained fighters or freemen peasantry (not serfs) or they would pay to equip or have these men equiped and that would satisfy their obligation to their lord.
**Basically, nobles could get out of fighting by paying if they had the money, there was often a set standard, as in: For this property, this noble owes 10 knights and 25 men at arms with helmets, swords and shields, and 25 archers.
*There are instances where nobles owing 'duel fealty' as Vomaxx stated, paid both lords, or paid one and took the field in person against the other! Wild huh? The middle ages rocked.
As Vomaxx already said squires were very rarely low-born. As in VERY RARELY! They were the sons of nobility who weren't the lads seeking futures in the church as clerics, etc. An average income noble with many sons was expected to split up the lot and often it was the second son who went to serve the church. The rest could end up with various lords in the kingdom, serving high lords (hopefully) or knights if pop was lucky to place them well and have them learn knightly ways.
*Low born squires in actual documented history are so rare that they were always special circumstances individuals. Special circumstances being anything from divine inspiration (Like Joan de Arc - the low-born French girl who ended up influencing the King of France and co-leading his armies) to merchants (low-born) who did well enough in trade who could pay poorer knights to have their sons taken in as squires. None of this happened often. The low born simply weren't placed in the same social class as the nobility and were expected to serve them at best, but were often seen ranking below a good hunting dog. They were cattle.
okay, lecture over... sorry