The nearest thing to this in the English-speaking rugby world is a development contract. But the French "espoir" contract has its own distinctive features and I would recommend adding the French term as well. Discussions of the French system in rugby circles often use the French term.

Here's the definition from the French rugby collective agreement:

"Le contrat espoir est un contrat de travail à durée déterminée dit « d’usage » par application des articles L. 1242-2-3° et D. 1242-1 du Code du travail, par lequel un Club rémunère un joueur âgé de 16 à 23 ans en contrepartie de la pratique du rugby tout en lui permettant de poursuivre une formation sportive, ainsi qu’une formation professionnelle, scolaire ou universitaire méthodique, complète et continue dans le cadre du centre de formation du Club."
http://www.lnr.fr/IMG/pdf/CCRP_v_consolidee_au_29_avril_2014...

The only other kinds of contract for players in the French Top 14 are "professionnel" and "pluriactif".

It's not an academy contract, which is called a convention de formation in French. The next step up from that in British, Irish and Southern Hemisphere rugby is the development contract.

"Upon joining UBB, the Lautoka native signed a two-year espoir contract (similar to a development contract). That deal expires next June and the likes of Toulouse, Clermont, Toulon and Leicester have already cast lustful glances in Talebula’s direction."
https://murraykinsella.wordpress.com/category/french-rugby/

"Toulouse regularly play development 'Espoir' players in T14 games.
These are young players not on pro contract ."
http://www.scumv.co.uk/viewtopic.php?f=32&t=51157&start=15


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Note added at 1 hr (2015-09-30 09:12:47 GMT)
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If you look for "rugby" "development contract" you'll find stacks of references. It's the standard term for a junior player's contract between an academy contract and a full professional contract.

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Note added at 1 hr (2015-09-30 09:19:43 GMT)
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"Instead of being dragged into the Scotland amateur system, his agent bagged him an 'Espoir' contract at Top 14 side Perpignan."
http://en.espn.co.uk/italy/rugby/story/205017.html

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Note added at 1 hr (2015-09-30 09:30:25 GMT)
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For promising young players these contracts are an attractive option, with reasonable pay and first-team play and prospects of a full professional contract to follow, with luck. For the big clubs one of their great attractions is that they don't count in the salary cap (the club's maximum permitted salary bill).